Remove the unneeded escaping of '[' and ']' characters in test_class_help

Remove the unneeded escaping of '[' and ']' characters in test_class_help

As these characters don't need to be escaped for strings
wrapped inside {} braces, we can remove the unneeded backslashes.

gdb/testsuite/ChangeLog:

	* lib/gdb.exp (test_class_help): Remove the unneeded escaping of
	'[' and ']' characters.