Don't try to lex Python code in the Qt console for completion

This addresses part of #6726 - completions on function keyword args and
filenames in strings now work again.
Remove unused completion lexer code

_handle_complete_reply() in FrontendWidget is overridden in
IPythonWidget, so it's never used, and it would presumably now be wrong
if it ever was used.

There is no longer any need for lexing to work out what to complete: we
just send the whole input buffer to the kernel.
Merge pull request #6728 from takluyver/qtconsole-completion

Fix completion things in the Qt console