[FIX] core: fix handling of removals in sub-o2m

Issue is specifically in the case of an onchange removing a record in
an o2m in an o2m (so a sub-o2m) if loading an *existing* record in the
SSF: since the server pretty much only returns a REMOVE_ALL followed
by the records to keep or create, conserving the removal information
requires diffing the value currently stored in the form and the result
fo the onchange.

Diff which was properly done for top-level o2ms, but not for the ones
below that (apparently forgot this bit when improving support for
nested o2ms earlier this year).

X-original-commit: 177d0095