Remove 64bit variant of software codecs

Since mediacodec is a 32-bit process, we don't need the 64bit variant of
the software codecs. This will reduce the system partition usage by
about 11MB (for devices with VNDK) or 5.8MB (for other devices).

Bug: 64193930
Test: no libstagefright_soft_*.so files under /system/lib64/ and
/system/lib64/vndk directories
Test: YouTube, Camcorder recording, Video playback work

Change-Id: I52d4947c673c018ac094b3c84c1652d217611894