Revert addition of electric-block-comment-mode & follow-up commits

Revert addition of electric-block-comment-mode & follow-up commits

As presently under discussion in bug#77823, the intended new
functionality is not really about comments at all.
Remove it for now to allow us to redesign from a clean slate,
and to deal with the regression reported in bug#77823.

This reverts the following three changesets:

Author:     Elías Gabriel Pérez <eg642616@gmail.com>
AuthorDate: Mon Mar 17 12:56:52 2025 -0600

  New minor mode: `electric-block-comment-mode'

Author:     Elías Gabriel Pérez <eg642616@gmail.com>
AuthorDate: Mon Mar 31 17:58:16 2025 -0600

  Add block-comment-start and block-comment-end to supported modes

Author:     Elías Gabriel Pérez <eg642616@gmail.com>
AuthorDate: Sun Apr 13 12:26:08 2025 -0600

  Add block-comment variables to cc-mode

(cherry picked from commit 4808f785ccb0166a87983405c6317ab2a4d6dfde)