Maintenance: remove outdated macro AC_REVISION (#2344)

The value of this has not been meaningful since
we stopped committing ./configure to VCS
over 15 years ago.

Author: yadij

configure.ac

@@ -11,7 +11,6 @@ AC_CONFIG_HEADERS([include/autoconf.h])
 AC_CONFIG_AUX_DIR(cfgaux)
 AC_CONFIG_SRCDIR([src/main.cc])
 AM_INIT_AUTOMAKE([tar-ustar nostdinc subdir-objects dist-xz])
-AC_REVISION($Revision$)dnl
 AC_PREFIX_DEFAULT(/usr/local/squid)
 AM_MAINTAINER_MODE