Get rid of the INFO_EXT variable

Get rid of the INFO_EXT variable
It's never been anything more than pointless complexity

* configure.ac (INFO_EXT, INFO_OPTS): Remove output variables.

* Makefile.in (INFO_EXT): Remove and replace by ".info" throughout.

* doc/emacs/Makefile.in (INFO_EXT): Remove and replace by ".info" throughout.
(INFO_OPTS): Set directly rather than with configure.

* doc/lispintro/Makefile.in (INFO_EXT): Remove and replace by ".info" throughout.
(INFO_OPTS): Set directly rather than with configure.

* doc/lispref/Makefile.in (INFO_EXT): Remove and replace by ".info" throughout.
(INFO_OPTS): Set directly rather than with configure.

* doc/misc/Makefile.in (INFO_EXT): Remove and replace by ".info" throughout.
(INFO_OPTS): Set directly rather than with configure.