Fixes a regression introduced in b7c7c03eb5.
* guix/build/emacs-build-system.scm (package-name-version->elpa-name-version):
Remove unused 'name' binding. Do not abuse 'strip-store-file-name' to
remove the "emacs-" prefix, which worked before b7c7c03eb5 but not after.
* 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'.