Use hook instead of face list to inhibit electric quoting

Use hook instead of face list to inhibit electric quoting

This is more flexible and doesn't couple electric quoting to font
locking.
Give that 'electric-quote-code-faces' was just introduced, remove it
without formal deprecation.

* lisp/electric.el (electric-quote-inhibit-functions): New abnormal
hook variable.
(electric-quote-post-self-insert-function): Run the hook.  Remove
use of old 'electric-quote-code-faces' variable.

* test/lisp/electric-tests.el (electric-quote-markdown-in-text)
(electric-quote-markdown-in-code): Adapt unit tests.