[gdb/testsuite] Remove unused globals

Remove unused global variable declarations.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-02-27  Tom de Vries  <tdevries@suse.de>

	* config/sid.exp: Remove unused globals.
	* gdb.base/attach.exp: Same.
	* gdb.base/catch-load.exp: Same.
	* gdb.base/dbx.exp: Same.
	* lib/gdb.exp: Same.
	* lib/mi-support.exp: Same.
	* lib/prompt.exp: Same.