InlineSchemaTree: don't resolve subpointers anymore

This is illegal, after all. Either we are using JSON Pointer, or we are not.

The new logic is now:

* if the id _is_ an absolute JSON Reference, then JSON Pointer may be used;
* otherwise it is never used at all, and an exact ref match is required.

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>