[gdb/testsuite] Remove unnecessary warning in gdb.base/default.exp

When running test-case gdb.base/default.exp with target board
native-gdbserver, we get:
...
WARNING: Skipping backtrace and break tests because of GDB stub.
...

There's no need for such a warning, so remove it.

Tested on x86_64-linux with native and target board native-gdbserver.