[editors/emacs] Fix protobuf-mode definition (#9572)

Fix a few issues with the Emacs mode definition.

First, in 1ab7789f3 (2021-10-13, Emacs: Protobuf mode should be derived from
prog-mode) we made `protobuf-mode' a derived mode of `prog-mode' using the
`define-derived-mode' macro[1]. However, the definition body was not updated
accordingly. So in this commit, we:

- Remove the superfluous `(interactive)' form;
- Remove the unnecessary call of `kill-all-local-variables', which is already
  handled by `define-derived-mode' and could cause a few issues, for example,
  it prevents `prog-mode-hook' from being run;
- Remove forms that set `major-mode' and `mode-name', which are automatically
  set to the first and third arguments respectively;
- Remove forms that set key map, syntax table, and abbrev table, which are
  already handled automatically;
- Do not run `protobuf-mode-hook' explicitly in the body. It is already arranged
  to be run after the body.

Second, the call to `c-make-emacs-variables-local' is removed. It is called
inside `c-init-language-vars' already. Calling it again should do no harm now,
but to be future-proof it might be better to just remove it.

Finally, we move the `c-update-modeline' form to the :after-hook argument to
ensure it is run at the very end, so that the mode line will reflect all user
customizations done in various mode hooks. Similarly, we run
`c-mode-common-hook' also at the very end to leave a place for user
customizations (for example, set `imenu-generic-expression' in
`c-mode-common-hook').

[1] https://www.gnu.org/software/emacs/manual/html_node/elisp/Derived-Modes.html

Re: https://github.com/protocolbuffers/protobuf/issues/7316
Re: https://github.com/protocolbuffers/protobuf/pull/9076