Revert "Handle PERM_EXPORT in LLPermissions."

This reverts commit 9c71afe73e80209e0c5ab6d3911975cea1c326f1.
Move export permission check to LLPermissions, try two.

Removes code duplication.

Several parts of the code ignored the PERM_EXPORT but
and still did demand that things are full perm next
to being a creator. This has now changed the export
rules are the same for everything as they were for mesh:
you need to be the owner and the creator for every element
that is exported (not just the root prim, of course).