Avoid trying to add already-cached selectors to reject cache.  Remove pre-insertion search.
Avoid trying to add already-cached selectors to reject cache.  Remove pre-insertion search.

svn path=/trunk/libcss/; revision=13131

prepare/netsurf_libcss,zhuyadong/libcss,servo/libcss,zhuyadong/libcss,prepare/netsurf_libcss,servo/libcss,servo/libcss,prepare/netsurf_libcss,zhuyadong/libcss