[alsa] Fix multiple find_package(ALSA REQUIRED)

[dg0yt]

Resolves #42929.

[BillyONeal]

Just to make sure I understand, there are 2 changes here:

1. Removing what is effectively a debug check that probably shouldn't run during a user's configure.
2. "Promoting" ALSA_LIBRARY to a cache variable.

and those are really independent changes?

[dg0yt]

Yes, those are two independent changes.

[BillyONeal]

Thanks!