virt-manager: remove quartz variant - fix lint warnings

The Quartz variant existed to provide a patch which is no longer
needed.

gnome/virt-manager/Portfile

@@ -31,6 +31,8 @@ checksums           rmd160  720822cf863071b646a02fc487ea0082e5959097 \
                     sha256  ee889d59110986391a394077f004f68125e01e216a5e7cfc29adb6ae49ab2dab \
                     size    2796831
 
+platforms           darwin
+
 python.default_version  27
 
 post-patch {
@@ -55,12 +57,6 @@ depends_run \
     port:spice-gtk \
     port:libosinfo
 
-variant quartz {
-    # forking at launch crashes 100% of the time - it should work in
-    # X11, but I haven't checked
-    patchfiles-append suppress-fork.diff
-}
-
 use_configure       yes
 configure.cmd       ${build.cmd} configure
 configure.args       --prefix ${python.prefix}