darshan-runtime.pc.in - remove requirements of mpich and zlib

Darshan may be built with a different MPI library.
Also, it may be built without zlib.

Author: wkliao

darshan-runtime/pkgconfig/darshan-runtime.pc.in

@@ -5,7 +5,7 @@ Name: darshan
 Description: Darshan runtime library
 Version: @DARSHAN_VERSION@
 
-Requires: zlib mpich
+Requires:
 Requires.private:
 
 prefix=@prefix@