Remove obsolete vendor code from Alpine platform support

This is a clean-up patch from a serie delivering support for
Annapurna Labs Alpine PoC.
The HAL files have already been added to sys/contrib/alpine-hal
so there is no need for them in the platform directory.
This patch removes obsolete files.

Reviewed by: andrew
Obtained from: Semihalf
Sponsored by: Annapurna Labs
Differential Revision: https://reviews.freebsd.org/D3248