Remove V2 "library types" and associated logic

[kdmccormick]

In an older iteration of V2 content libraries, we thought we'd have different "types" of libraries: video libraries, problem libs, and complex (mixed/advance) libraries.

In the V2 libraries relaunch, libraries will not have types, so we can remove all the associated logic.

Remove these constants, defined in openedx/core/djangoapps/content_libraries/constants.py:

VIDEO = 'video'
COMPLEX = 'complex'
PROBLEM = 'problem'

LIBRARY_TYPES = (
    (VIDEO, _('Video')),
    (COMPLEX, _('Complex')),
    (PROBLEM, _('Problem')),
)

Remove every bit of code that references those constants or otherwise references libraries types. Grep for library_type.

This will involve backwards-incompat HTTP API changes, but that's OK since the APIs were never officially supported.