Remove VARNISH_PREREQ in configure.ac for the master branch.

This allows for builds against Varnish trunk or any numbered
version; but it doesn't check for a minimum version. In branch 6.0,
we have VARNISH_PREREQ([6.0.0]), which can check against numbered
versions, but fails for Varnish trunk.

configure.ac

@@ -30,7 +30,6 @@ AC_ARG_WITH([rst2man],
 
 m4_ifndef([VARNISH_PREREQ], AC_MSG_ERROR([Need varnish.m4 -- see INSTALL.rst]))
 
-VARNISH_PREREQ([master])
 VARNISH_VMODS([dispatch])
 
 VMOD_TESTS="$(cd $srcdir/src && echo tests/*.vtc)"