Do not depend on module 'ag-common'

'ag-common' is the old name of dune-fufem, and has gone out of use
years ago.  And dune-tnnmg doesn't depend on dune-fufem anyway.

Thanks to Andreas Dedner for spotting this.

dune-tnnmg.pc.in

@@ -10,6 +10,6 @@ Name: @PACKAGE_NAME@
 Version: @VERSION@
 Description: dune-tnnmg module
 URL: http://dune-project.org/
-Requires: dune-common dune-grid dune-istl dune-solvers ag-common
+Requires: dune-common dune-grid dune-istl dune-solvers
 Libs: -L
 Cflags: -I