[gdb/testsuite] Remove duplicate of supports_gnuc

In commit 9d9dd861 ("[gdb/testsuite] Fix regression in
step-indirect-call-thunk.exp with gcc 7") I accidentally committed a duplicate
of supports_gnuc, which caused:
...
DUPLICATE: gdb.base/gdb-caching-proc.exp: supports_gnuc: consistency
...

Fix this by removing the duplicate.

Tested on x86_64-linux.