testsuite: Cleanup some temp dirs with gdb-index files

After the gdb test-suite runs there are some files
left in /tmp/tmp*/*.gdb-index, remove those files
and the directory at the end of the test case.

gdb/testsuite:
2021-05-14  Bernd Edlinger  <bernd.edlinger@hotmail.de>

	* gdb.base/index-cache.exp: Cleanup $cache_dir/*.gdb-index and
	remove the directory.
	* gdb.dwarf2/per-bfd-sharing.exp: Likewise.