utils: Add 'strip-store-file-name'.

* guix/build/utils.scm (strip-store-file-name): New procedure.
* guix/build/emacs-build-system.scm (store-directory->name-version):
  Remove.  Update callers to use 'strip-store-file-name'.
* gnu/packages/gcc.scm (make-libstdc++-doc)[arguments]: Use
  'strip-store-file-name' instead of 'string-drop'.