Revert "Add emacs keybindings for mark emulation" (#23146)

- Reverts zed-industries/zed#22904
- See also: https://github.com/zed-industries/zed/issues/8580

After using it full-time for a day I very much think an implicit "mark
mode" when the emacs base keymap is enabled is the wrong approach.

Release Notes:

- Reverted "Add emacs keybindings for mark emulation" #23146 (main only)