proxy.cgi: Remove option to show Squid's version entirely

proxy.cgi: Remove option to show Squid's version entirely

There is no sense to display this to anybody, and we do not reveal
version information anywhere else on purpose. The IT staff knows which
version of IPFire they are running (hopefully the latest), and it's
none of the rest of the world's business.

Fixes: #12665 (in some way)
Signed-off-by: Peter Müller <peter.mueller@ipfire.org>
Acked-by: Michael Tremer <michael.tremer@ipfire.org>
Signed-off-by: Arne Fitzenreiter <arne_f@ipfire.org>