Fixes for the FluidSynth plugin
fluiddec: Avoid deprecated fluid_synth_set_sample_rate
This function is used to change the rate at runtime, which has issues:
https://github.com/FluidSynth/fluidsynth/issues/585
Use the settings key instead (which already defaults to 44100, but I did
test other rates).
fluiddec: Keep fluidsynth from probing audio drivers
It might cause problems and we don't need the drivers anyway. This also
avoids a bunch of stderr spam from the drivers.