Retire the MPVST_AUDIOIF_LIB alias and MPVST_LIB_RETIRED after the next release

[bdbarnett]

Follow-up to the audioif -> audiocomponents retarget (audiocomponents#2).

• 4c3ab54 renamed MPVST_AUDIOIF_LIB to MPVST_COMPONENTS_LIB in
  src/plugin/CMakeLists.txt; the old name is still honoured, with a
  warning, when it is set and the new one is not (a stale CMake cache
  sets it).
• aad1f5a did the same for the environment variable in tools/piece.py
  (stderr notice).
• 4c3ab54 also added MPVST_LIB_RETIRED to stage_lib.cmake, so a
  build directory that staged from audioif/lib before the move drops
  its orphaned audiorender/ and stamp.

All three were promised for one release. After the first release that
ships with the rename, remove:

• the MPVST_AUDIOIF_LIB alias block in src/plugin/CMakeLists.txt
• _COMPONENTS_LIB_ALIAS in tools/piece.py
• the -DMPVST_LIB_RETIRED argument in src/plugin/CMakeLists.txt and
  the foreach(retired ...) pass in src/plugin/stage_lib.cmake
• the README sentences about MPVST_AUDIOIF_LIB and -U MPVST_AUDIOIF_LIB