libmpeg2: switch SIMD selection to auto-detect mode

this:
 - enables SSE2 (when available at runtime), which was added 13 years ago
   ([1]) but we never enabled use of it until now.
 - removes use of the MMX/MMXEXT availability testing functions, helping
   pave the way towards our goal of purging all MMX/MMXEXT code.
 - makes the code cleaner and reduces maintenance burden.
 - allows us to pick up use of new SIMD variant additions, if any more are
   ever added (unlikely) without having to explicitly add code to enable it.

as pointed out in review, the `mpeg2_init()` call makes a call to
`mpeg2_accel()` with `MPEG2_ACCEL_DETECT` itself, so there is no need to
make such a call ourselves, thus the `mpeg2_init()` call alone is
sufficient. the entire code block can thus be safely removed.

[1]: videolan/libmpeg2@c80d1dc2