ALSA: hda/realtek - Use common helper for creating beep controls

In the Realtek codec driver, we used to build kctl elements for beep
mixer in the own build_controls callback.  This is an open-code and
can be covered by the standard feature of the generic parser with
snd_hda_gen_add_kctl() instead.

Also, after the conversion, spec->beep_amp becomes superfluous; hence
it's removed along with the conversion.

Signed-off-by: Takashi Iwai <tiwai@suse.de>