audio/alsa-plugins: make JACK independent from SAMPLERATE

Dates back to 4070922aacfb but the submitted patches unconditionally
passed --disable-samplerate, so SAMPLERATE option was added by fluffy@.
According to v1.0.23 vendor sources there was no such dependency.
Drop as JACK=on SAMPLERATE=off builds fine, and neither 50-jack.conf
nor alsa-lib/files/asound.conf.sample use samplerate.