nixos/doc: don't prescribe OS / manual name

In general the man pages do not care what OS and manual they are shipped
with, so they don't get to choose the names of them. We were tempted to
do so, as we had inconsistently chosen OS names for the mandoc and
man-db/groff implementations. Since this has been rectified since, we
can just drop this boilerplate from the man pages.

nixos/doc/manual/manpages/nixos-build-vms.8

@@ -1,10 +1,6 @@
 .Dd January 1, 1980
-.\" nixpkgs groff will use Nixpkgs as the OS in the title by default, taking it from
-.\" doc-default-operating-system. mandoc doesn't have this register set by default,
-.\" so we can use it as a groff/mandoc switch.
-.ie ddoc-default-operating-system .Dt nixos-build-vms \&8 "NixOS System Manager's Manual"
-.el .Dt nixos-build-vms 8
-.Os NixOS
+.Dt nixos-build-vms 8
+.Os
 .Sh NAME
 .Nm nixos-build-vms
 .Nd build a network of virtual machines from a network of NixOS configurations

nixos/doc/manual/manpages/nixos-enter.8

@@ -1,10 +1,6 @@
 .Dd January 1, 1980
-.\" nixpkgs groff will use Nixpkgs as the OS in the title by default, taking it from
-.\" doc-default-operating-system. mandoc doesn't have this register set by default,
-.\" so we can use it as a groff/mandoc switch.
-.ie ddoc-default-operating-system .Dt nixos-enter \&8 "NixOS System Manager's Manual"
-.el .Dt nixos-enter 8
-.Os NixOS
+.Dt nixos-enter 8
+.Os
 .Sh NAME
 .Nm nixos-enter
 .Nd run a command in a NixOS chroot environment

nixos/doc/manual/manpages/nixos-generate-config.8

@@ -1,10 +1,6 @@
 .Dd January 1, 1980
-.\" nixpkgs groff will use Nixpkgs as the OS in the title by default, taking it from
-.\" doc-default-operating-system. mandoc doesn't have this register set by default,
-.\" so we can use it as a groff/mandoc switch.
-.ie ddoc-default-operating-system .Dt nixos-generate-config \&8 "NixOS System Manager's Manual"
-.el .Dt nixos-generate-config 8
-.Os NixOS
+.Dt nixos-generate-config 8
+.Os
 .Sh NAME
 .Nm nixos-generate-config
 .Nd generate NixOS configuration modules

nixos/doc/manual/manpages/nixos-install.8

@@ -1,10 +1,6 @@
 .Dd January 1, 1980
-.\" nixpkgs groff will use Nixpkgs as the OS in the title by default, taking it from
-.\" doc-default-operating-system. mandoc doesn't have this register set by default,
-.\" so we can use it as a groff/mandoc switch.
-.ie ddoc-default-operating-system .Dt nixos-install \&8 "NixOS System Manager's Manual"
-.el .Dt nixos-install 8
-.Os NixOS
+.Dt nixos-install 8
+.Os
 .Sh NAME
 .Nm nixos-install
 .Nd install bootloader and NixOS

nixos/doc/manual/manpages/nixos-option.8

@@ -1,10 +1,6 @@
 .Dd January 1, 1980
-.\" nixpkgs groff will use Nixpkgs as the OS in the title by default, taking it from
-.\" doc-default-operating-system. mandoc doesn't have this register set by default,
-.\" so we can use it as a groff/mandoc switch.
-.ie ddoc-default-operating-system .Dt nixos-option \&8 "NixOS System Manager's Manual"
-.el .Dt nixos-option 8
-.Os NixOS
+.Dt nixos-option 8
+.Os
 .Sh NAME
 .Nm nixos-option
 .Nd inspect a NixOS configuration

nixos/doc/manual/manpages/nixos-rebuild.8

@@ -1,10 +1,6 @@
 .Dd January 1, 1980
-.\" nixpkgs groff will use Nixpkgs the OS in the title by default, taking it from
-.\" doc-default-operating-system. mandoc doesn't have this register set by default,
-.\" so we can use it as a groff/mandoc switch.
-.ie ddoc-default-operating-system .Dt nixos-rebuild \&8 "NixOS System Manager's Manual"
-.el .Dt nixos-rebuild 8
-.Os NixOS
+.Dt nixos-rebuild 8
+.Os
 .Sh NAME
 .Nm nixos-rebuild
 .Nd reconfigure a NixOS machine

nixos/doc/manual/manpages/nixos-version.8

@@ -1,10 +1,6 @@
 .Dd January 1, 1980
-.\" nixpkgs groff will use Nixpkgs as the OS in the title by default, taking it from
-.\" doc-default-operating-system. mandoc doesn't have this register set by default,
-.\" so we can use it as a groff/mandoc switch.
-.ie ddoc-default-operating-system .Dt nixos-version \&8 "NixOS System Manager's Manual"
-.el .Dt nixos-version 8
-.Os NixOS
+.Dt nixos-version 8
+.Os
 .Sh NAME
 .Nm nixos-version
 .Nd show the NixOS version