As recently discussed on the emacs-devel list, I'd like to eradicate
conditional code from cperl-mode where the conditionals evaluate to nil.

This is a rather large part of the cleanup, related to all that stuff
which has changed in the last years with regard to font-lock-mode and
cperl-mode's homegrown fontification based on features which are no
longer available.

I tested Perl code on my repositories (twiki.org code and
https://github.com/act-psgi/Act), written by different authors in
different styles, and could not find any change in fontification.

The conditionals leading to dead code are:

 - (featurep 'choose-color) is nil
 - (featurep 'font-lock-extra) is nil
 - (facep 'font-lock-constant-face) is t and
   (boundp 'font-lock-constant-face) is t
   ...same for other faces provided by font-lock-mode.

Once this dead code is eliminated, the macro cperl-force-face and the
function cperl-init-faces-weak (both undocumented) lose their purpose
and are also deleted.

There should be no user-visible changes, so I have made no entry in
etc/NEWS.
--
Cheers,
haj