Add method `remove(JsonPointer)` in `ContainerNode`

[cowtowncoder]

(note: related to #1980)

With addition of JsonPointer, it seems reasonable to allow removal of a node pointed to by expression.

Addition of a method that returns JsonNode removed (or null to indicate none) is simple enough.
But one question is whether this should be added in JsonNode (more convenient) or only in ContainerNode (which is philosophically better, given JsonNode currently only contains read-methods and no modifications).

[cowtowncoder]

Alas, did not make it in 3.0 -- maybe 3.1