Remove language_demangle

Remove language_demangle

I noticed that language_demangle shadows the global
"current_language".  When I went to fix this, though, I then saw that
language_demangle is only called in two places, and has a comment
saying it should be removed.  This patch removes it.  Note that the
NULL check in language_demangle is not needed by either of the
existing callers.

Regression tested on x86-64 Fedora 36.

Approved-By: Simon Marchi <simon.marchi@efficios.com>