Remove some unneeded build deps
... that were added in commit c038f5e5b56d657cfc7ba32446de11df2b37b61b
without any explanation. gnome-icon-theme build doesn't need the header
files from gtk2-devel and librsvg2-devel and only needs the base
packages -- the gtk2 build dep is solely for
/usr/bin/gtk-update-icon-cache.
https://bugzilla.redhat.com/show_bug.cgi?id=972372#c9