Blender Git Loki

Fix T88498: 'Clear Parent' does not clear parent_bone

Clearing the parent from the UI using the X (or from python) clears the
`parsubstr` and set `partype` back to `PAROBJECT`.

Using the Clear Parent operator would leave the `parsubstr` (and thus
`parent_bone`) untouched even though this operator claims to "clear
parenting relationship completely" (it also removes parent deform
modifiers for example).

So now, also clear `parsubstr` and set back to `PAROBJECT` [which is
default].

Maniphest Tasks: T88498

Differential Revision: https://developer.blender.org/D11503