cargo: nuke 'simd-accel' from orbit

[BurntSushi]

This feature causes nothing but problems and is frequently broken. The
only optimization it was enabling were SIMD optimizations for
transcoding. In particular, for UTF-16 transcoding. This is performed by
the encoding_rs crate,
which specifically uses unstable portable SIMD APIs instead of the
stable non-portable SIMD APIs.

SIMD optimizations that apply to search have long been making use of
stable APIs, and are automatically enabled when your target supports
them. This is, IMO, the correct user experience and one that
encoding_rs refuses to support. I'm done dealing with it, so
transcoding will only use scalar code until the SIMD optimizations in
encoding_rs work on stable. (This doesn't mean that encoding_rs has
to change. This could also be fixed by stabilizing std::simd.)

Fixes #2748