cperl-mode: Delete conditional code where conditions evaluate to nil

cperl-mode: Delete conditional code where conditions evaluate to nil

* lisp/progmodes/cperl-mode.el (cperl-force-face): This
macro's single effect is now inlined, and the macro is gone.
(cperl-problems): The reference to choose-color.el, which
is no longer available for download, is deleted.
(no function): A list of unnecessary empty variable
definitions is gone.  They were needed for Emacs v19 and
below.
(cperl-init-faces-weak): This function does no longer do
anything and is therefore deleted.
(cperl-init-faces): Some bodies of conditional code is deleted
because as of today the conditions evaluate to constants.  The
face cperl-nonoverridable-face is no longer available as
variable and needs to be doubly-quoted in one place (bug#43622).