CppTools: Do not offer pointless ")" signature completion

If "Automatically insert matching characters" is activated and '(' is
typed before the semicolon in the declaration

    int f;

we offered the ')' completion, although the ')' was already inserted.

Change-Id: Id5a50fa94c2cb7b5c4056d87028501b0b90afc73
Reviewed-by: Erik Verbruggen <erik.verbruggen@theqtcompany.com>