gnu: emacs-stream: Don't set a source file name.

This is a follow-up to 678248f5ea.

* gnu/packages/emacs-xyz.scm (emacs-stream)[source]: Remove FILE-NAME, as the
tarball name is already OK; and the -checkout name breaks build system
assumptions on the core-updates branch.

gnu/packages/emacs-xyz.scm

@@ -12665,7 +12665,6 @@ editing nginx config files.")
        (method url-fetch)
        (uri (string-append "https://elpa.gnu.org/packages/"
                            "stream-" version ".tar"))
-       (file-name (git-file-name name version))
        (sha256
         (base32 "00c3n4gyxzv7vczqms0d62kl8zsmjfyxa92mwxn2snyx857a9jfw"))))
     (build-system emacs-build-system)