Forms: when opening block picker in context of Form, remove "field" suffix

Small but if this is easy meaningful polish — we should remove the "field" suffix when opening the picker in context of Forms:

"Field" suffix is still important when adding individual fields from top level block picker outside Forms.

Came up as feedback from Matías in p1HpG7-l0G-p2#comment-61251

Search the Jetpack repository for the Forms block picker and the label that adds the "field" suffix. Compare the picker opened from Forms with the top-level picker, then verify that only the Forms context omits the suffix while individual fields retain it; check the nearby frontend tests if available.

Written by the indexing model from the issue text.