libi386: remove BD_SUPPORT_FRAGS

BD_SUPPORT_FRAGS is preprocessor knob to allow partial reads in bioscd/biosdisk
level. However, we already have support for partial reads in bcache, and there
is no need to have duplication via preprocessor controls.

Note that bioscd/biosdisk interface is assumed to perform IO in 512B blocks,
so the only translation we have to do is 512 <-> native block size.

Differential Revision: https://reviews.freebsd.org/D16600