mpich: Fix +vci variant for newer releases (#42570)

--with-ch4-max-vcis=default is no longer accepted by MPICH configure
since the 4.1 release. Just omit the option from the +vci variant, since
configure will select the default value in its absence.

var/spack/repos/builtin/packages/mpich/package.py

@@ -620,7 +620,6 @@ def configure_args(self):
 
         if "+vci" in spec:
             config_args.append("--enable-thread-cs=per-vci")
-            config_args.append("--with-ch4-max-vcis=default")
 
         if "datatype-engine=yaksa" in spec:
             config_args.append("--with-datatype-engine=yaksa")