stand/zfs: Remove read-compatible features

These features are marked as ZFEATURE_FLAG_READONLY_COMPAT and so
irrelevant for read-only pool imports by the loader:

"com.datto:resilver_defer",
"com.delphix:obsolete_counts",
"com.delphix:spacemap_histogram",
"com.delphix:zpool_checkpoint",
"com.intel:allocation_classes",
"org.zfsonlinux:allocation_classes"

This should cause no functional changes, just a code cleanup.

MFC after: 2 months