Remove support for AmigaOS

Remove support for AmigaOS

There is a lot of specialized code for supporting AmigaOS and it has
not been maintained for a number of years.  It's highly unlikely that
the latest versions even compile properly on AmigaOS anymore.  After
requesting that someone step forward to own the maintenance of the
port in the GNU Make 4.4 release and getting no takers, I removed it.

* NEWS: Announce the removal.
* README.in: Remove README.Amiga reference.
* README.Amiga: Remove unused file.
* SCOPTIONS: Ditto.
* src/amiga.c: Ditto.
* src/amiga.h: Ditto.
* src/config.ami: Ditto.
* mk/Amiga.mk: Ditto.
* Makefile.am: Remove references to deleted files.
* Basic.mk.template: Ditto.
* maintMakefile: Ditto.
* src/commands.c: Remove ifdef'd Amiga code.
* src/default.c: Ditto.
* src/dir.c: Ditto.
* src/file.c: Ditto.
* src/function.c: Ditto.
* src/job.c: Ditto.
* src/job.h: Ditto.
* src/main.c: Ditto.
* src/os.h: Ditto.
* src/read.c: Ditto.
* src/remake.c: Ditto.