Retire CLANG_IS_CC option

A small reduction in build infrastructure complexity; when we had both
Clang and GCC in the tree it was useful to have both built, and choose
one or the other to install as /usr/bin/cc.  Now only Clang is in the
tree, and there is no point in building and installing base Clang but
not providing it as cc (and c++, cpp).

Reviewed by:	imp
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D37075

UPDATING

@@ -27,6 +27,10 @@ NOTE TO PEOPLE WHO THINK THAT FreeBSD 14.x IS SLOW:
 	world, or to merely disable the most expensive debugging functionality
 	at runtime, run "ln -s 'abort:false,junk:false' /etc/malloc.conf".)
 
+20221121:
+	The WITHOUT_CLANG_IS_CC option has been removed.  When Clang is enabled
+	it is always installed as /usr/bin/cc (and c++, cpp).
+
 20221026:
 	Some programs have been moved into separate packages. It is recommended
 	for pkgbase users to do: