[gdb/testsuite] Fix gdb.base/index-cache.exp with cc-with-gdb-index

With target board cc-with-gdb-index we run into:
...
FAIL: gdb.base/index-cache.exp: couldn't remove files in temporary cache dir
...

The problem is that there are no files to remove, because the index cache
isn't used, as indicated by $expecting_index_cache_use.

Fix this by only FAILing when $expecting_index_cache_use.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2021-08-01  Tom de Vries  <tdevries@suse.de>

	* gdb.base/index-cache.exp:
diff --git a/gdb/testsuite/gdb.base/index-cache.exp b/gdb/testsuite/gdb.base/index-cache.exp
index 0daf7bb..496145e 100644
--- a/gdb/testsuite/gdb.base/index-cache.exp
+++ b/gdb/testsuite/gdb.base/index-cache.exp
@@ -227,7 +227,7 @@
 test_cache_disabled $cache_dir "after populate"
 
 lassign [remote_exec host sh "-c \"rm $cache_dir/*.gdb-index\""] ret
-if { $ret != 0 } {
+if { $ret != 0 && $expecting_index_cache_use } {
     fail "couldn't remove files in temporary cache dir"
     return
 }