fix(gpg): use default home directory

on darwin, setting a custom gpg home directory results causes `nix flake
check` to fail when using my customized gpg-agent module. this does not
necessarily appear to be an issue with my implementation (though there
are certainly issues with my implementation), but appears to be a result
of a known nix language issue where regular expression matching behaves
differently between linux and darwin. the issue only presents itself
during *checks* for linux systems on darwin, which *should* be okay
since i'm not trying to *build* or run nixos modules or systemd services.

see NixOS/nix#1537

while that github issue and other issues/PRs referencing it provide
numerous examples of how one might work around the regex issue,
unfortunately the custom hashing functions in home-manager's gpg-agent
module are pretty opaque and i haven't a clue about where to begin
applying a possible fix.

fortunately, the error is easy to avoid by using the default gnupg home
directory. considering that the only real benefit to a custom home
directory is decluttering $HOME, the solution for my immediate needs is
clear. however, the underlying hash function logic will need to change
before my custom gpg-agent module can ever be merged upstream...

users/profiles/gnupg.nix

@@ -30,7 +30,6 @@ in lib.mkIf ("" != pgpPublicKey) {
 
   programs.gpg = {
     enable = true;
-    homedir = "${config.xdg.dataHome}/gnupg";
 
     scdaemonSettings = {
       disable-ccid = true;