gdb.base/commands.exp: Remove unused global references

gdb.base/commands.exp: Remove unused global references

There are a few unused references to the gdb_prompt global.

gdb/testsuite/ChangeLog:

	* gdb.base/commands.exp (gdbvar_simple_if_test,
	gdbvar_simple_if_test, gdbvar_complex_if_while_test,
	progvar_simple_if_test, progvar_simple_while_test,
	progvar_complex_if_while_test, user_defined_command_test,
	user_defined_command_args_eval,
	user_defined_command_args_stack_test,
	user_defined_command_manyargs_test, bp_deleted_in_command_test,
	temporary_breakpoint_commands,
	gdb_test_no_prompt, redefine_hook_test,
	redefine_backtrace_test): Remove "global gdb_prompt".