gnu: gdb: Remove headers and libraries already in Binutils.

* gnu/packages/gdb.scm (gdb)[arguments]: Add #:modules.  Rename
  'post-install' phase to 'remove-libs-already-in-binutils'.  Change it
  to compute the intersection of the set of headers and libraries of GDB
  vs. Binutils and to remove each of the files found in both.