stand/zfs: Remove one more read-compatible feature

This feature is marked as ZFEATURE_FLAG_READONLY_COMPAT and so
irrelevant for read-only pool imports by the loader:

"com.delphix:spacemap_v2"

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

I'm sorry, missed it in previous commit.

MFC after: 2 months

(cherry picked from commit c2588f5e066bbeb37e6bbdd5faf14320c60c1cbd)