profiles/desktop/gnome: default enable gnome-extra-apps[share] on systemd
Now that we have a separate systemd subprofile under targets/, we can
default-enable gnome-user-share again via systemd subprofile, keeping
gnome-base/gnome installable out of the box on non-systemd profiles and
defaulting to installing gnome-user-share on systemd profiles as before.
As gnome-user-share is for now systemd specific, move the apache USE
flag tweaks over to only systemd subprofile as well.
Signed-off-by: Mart Raudsepp <leio@gentoo.org>