Simplify some bibtex.el variable definitions

Simplify some bibtex.el variable definitions

* lisp/textmodes/bibtex.el (bibtex-include-OPTkey)
(bibtex-user-optional-fields, bibtex-BibTeX-entry-alist)
(bibtex-biblatex-entry-alist, bibtex-generate-url-list): Use :risky
tag instead of risky-local-variable property.

(bibtex-entry-format, bibtex-maintain-sorted-entries)
(bibtex-sort-entry-class, bibtex-dialect)
(bibtex-autokey-name-case-convert-function): Use :safe tag instead
of safe-local-variable property.

(bibtex-autokey-name-case-convert-function): Let custom function
default to 'identity', not 'ignore'.

(bibtex-strings, bibtex-reference-keys): Define with defvar-local
instead of defvar+make-variable-buffer-local.