Fix broken dependency in Emacs package descriptor

There is no such package as `flymake-mode`, though `flymake`
is bundled with every modern Emacs, and `erlang-flymake.el`
works with it.

The spurious broken dependency caused MELPA-built packages of
the erlang elisp code to be uninstallable:

https://github.com/milkypostman/melpa/issues/560