Don't make bibtex-unify-case-convert buffer-local

Don't make bibtex-unify-case-convert buffer-local

The :local tag does not currently work as intended (it results in a
default value of bibtex-unify-case-convert of nil rather than
identity), and no other bibtex.el user option is automatically
buffer-local, so revert this recent change.

For discussion, see the following emacs-devel thread:
https://lists.gnu.org/r/emacs-devel/2020-11/msg00734.html

* lisp/textmodes/bibtex.el (bibtex-unify-case-convert): Don't make
automatically buffer-local for consistency with other user options,
and because the :local tag doesn't have the intended results.