Raise an error when detecting old-style backquotes.

Raise an error when detecting old-style backquotes.

They have been deprecated for a decade now.

* src/lread.c (Fload): Don't use record_unwind_protect to warn about
old-style backquotes any more.  They now generate a hard error.
(read1): Signal an error when detecting old-style backquotes.  Remove
unused label.
(syms_of_lread): Remove unused internal variable
'lread--old-style-backquotes'.
(load_error_old_style_backquotes): Rename from
'load_warn_oldstyle_backquotes'.  Signal an error.

* lisp/emacs-lisp/bytecomp.el (byte-compile-from-buffer): Remove check
from byte compiler.  It isn't triggered any more.

* test/src/lread-tests.el (lread-tests--old-style-backquotes): Adapt
unit test.

* test/lisp/emacs-lisp/bytecomp-tests.el
(bytecomp-tests--old-style-backquotes)
(bytecomp-tests-function-put): Adapt unit tests.

* etc/NEWS: Document change.