gdb/testsuite: use build_executable in gdb.server/stop-reply-no-thread.exp

This test uses prepare_for_testing, then does a clean_restart for each
test configuration.  prepare_for_testing does a build_executable plus a
clean_restart.  So the clean_restart inside prepare_for_testing is done
for nothing.

Change prepare_for_testing to just build_executable to avoid the
unnecessary clean_restart.

gdb/testsuite/ChangeLog:

	* gdb.server/stop-reply-no-thread.exp: Use build_executable
	instead of prepare_for_testing.

Change-Id: I8b2a2e90353c57c39c49a3665083331b4882fdd0