build: stop distributing the README.GIT file

There is no need for normal users to build from git.  Do not give
the impression that that procedure is in any way recommended.

Makefile.am

@@ -6,7 +6,7 @@ if USE_COLOR
 SUBDIRS += syntax
 endif
 
-EXTRA_DIST = IMPROVEMENTS README.GIT
+EXTRA_DIST = IMPROVEMENTS
 
 ACLOCAL_AMFLAGS = -I m4