media-sound/audacity: fix libsamplerate/libsox dep

media-sound/audacity: fix libsamplerate/libsox dep

Starting with audacity 2.0.3, only *one* resampling library can be enabled at a
time.  Due to this, the way the USE flags are currently handled make it
impossible to use the libsoxr library.  This commit does two things to remedy
this:

- never enable libresample (which isn't recommended anymore), and
- drop the libsoxr USE flag so that the libsamplerate/libsoxr choice is
  controlled only through the libsamplerate USE flag, thus allowing only one
  choice, which reflects the way the build system works.

These two minimal changes make it possible to properly select between the two
recommended resampling libraries.

Gentoo-bug: 536208
Signed-off-by: Marc Joliet <marcec@gmx.de>