ALSA: synth: Remove empty init and exit

For a sake of code simplification, remove the init and the exit
entries that do nothing.

Notes for readers: actually it's OK to remove *both* init and exit,
but not OK to remove the exit entry.  By removing only the exit while
keeping init, the module becomes permanently loaded; i.e. you cannot
unload it any longer!

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