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
dc8844f8ef