Remove some unneeded build deps

... that were added in commit c038f5e5b5
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

gnome-icon-theme.spec

@@ -10,12 +10,10 @@ URL:        http://www.gnome.org
 Source0: http://download.gnome.org/sources/gnome-icon-theme/3.8/%{name}-%{version}.tar.xz
 Source1: legacy-icon-mapping.xml
 
-BuildRequires: gnome-common
+BuildRequires: gtk2
-BuildRequires: gtk2-devel
 BuildRequires: icon-naming-utils >= 0.8.7
 BuildRequires: intltool
-BuildRequires: librsvg2-devel
+BuildRequires: librsvg2
-BuildRequires: pkgconfig
 Requires: hicolor-icon-theme
 
 BuildArch: noarch