cperl-mode: Eliminating references to obsolete packages

[Harald Jörg]

Hello Emacs,

My history with Emacs development is rather short, so I'd like to
check some assumptions. cperl-mode.el has quite some conditionals
checking for features of which I assume that they are now either
completely dead, or always available:

 - (featurep 'choose-color): choose-color.el used to be downloadable
   from Ilya Zakharevich's web page in the 20th century, but it is
   gone now.  I suggest it is safe to assume the value is always nil.

 - (featurep 'font-lock-extra): I failed to find any reference where
   this came from and suggest also to assume the value is always nil.

 - (featurep 'mode-compile): It seems that mode-compile.el was never
   part of GNU Emacs, and it seems to be abandoned by now.  I suggest
   it should be eliminated.

 - (boundp 'font-lock-constant-face): - also for other faces.  I guess
   it is safe to assume this is always true, and the current list of
   available faces as visible in the info page "Faces for Font Lock"
   hasn't changed (much) since Emacs 26.  I'd be grateful for a
   pointer to recent changes in this face list!

I'd like to prepare a patch which removes all conditionals querying
these functions, the associate code branches, and related
customization variables.  But before I do so, I'd like to check with
people who have been contributing to Emacs longer than I have.

Are there any opinions or objections?
--
Cheers,
haj