Remove the unused unbind-all bytecode

Remove the unused unbind-all bytecode

It was implemented but never generated, originally intended for
TCO in the pre-lexbind era (which was semantically dubious anyway).
Removing it speeds up the interpreter because there is no longer any
need for the outermost `count` variable unless checking is enabled.

* lisp/emacs-lisp/bytecomp.el:
* lisp/emacs-lisp/comp.el (comp-limplify-lap-inst):
* src/bytecode.c (BYTE_CODES, exec_byte_code):
Remove definition and implementation of unbind-all, freeing up the opcode
for other purposes.