Remove the build-time option CLASH_DETECTION

Remove the build-time option CLASH_DETECTION

Every platform supports it, and the runtime option `create-lockfiles'
replaces it.

* configure.ac (CLASH_DETECTION): Remove option.

* lisp/files.el (lock-buffer, unlock-buffer, file-locked-p):
Remove fallback aliases, since they are always defined now.

* src/buffer.c (Frestore_buffer_modified_p, Fkill_buffer):
* src/emacs.c (shut_down_emacs):
* src/fileio.c (Finsert_file_contents, write_region):
* src/filelock.c (top-level, syms_of_filelock):
* src/insdel.c (prepare_to_modify_buffer_1):
CLASH_DETECTION is always defined now.

* admin/CPP-DEFINES: Remove CLASH_DETECTION.