gdb: Remove an unbalanced stray double quote from a comment

What appears to be a stray double quote character in std-operator.def
causes incorrect highlighting in my editor.

The quote was introduced in this commit:

    commit 858be34c5a03bb8973679ebf00d360182434dc00
    Date:   Mon Sep 4 20:21:15 2017 +0100

        Handle "p S::method()::static_var" in the C++ parser

I can't see any reason why the quote should be there, so this commit
removes it.

gdb/ChangeLog:

	* std-operator.def: Remove unbalanced, stray double quote
	character.