bug#52455: Remove non-working Emacs "support" for macOS unexec

[Paul Eggert]

I took a bleeding-edge GNU Emacs (commit 
85ca91ff912da7c3a672e4bfe6ce103a23ddec73 dated today) and tried to build 
it on cfarm104.cfarm.net, which is running macOS 12.6 21G115. I used 
this command to configure and build:

  ./configure --with-gnutls=ifavailable --with-unexec=yes
  make check

The build failed mysteriously.

After doing this, I discovered GNU bug#52455, which says that unexec 
builds have not worked in macOS for some time and that this bug won't be 
fixed. I wish I hadn't wasted my time trying --with-unexec.

I suggest removing support for unexec on macOS, so that nobody else 
wastes time like I did.