Remove legacy XModule mixins from xblocks-contrib XBlocks

[irtazaakram]

Description:

As per discussion here #151 (review)
XBlocks in xblocks-contrib still depend on old XModule-related mixins:

• XModuleMixin - Remove usage in xblocks-contrib, no changes to edx-platform.
  • The methods should be removed from xblocks-contrib and all the test in this repo.
  • If we get test failures in edx-platform, let's discuss and fix forward. It may involve adding XModuleMixin to content libraries.
• XModuleToXBlockMixin - Merge into each block using it.
• XmlMixin - Add parse_xml method to problem block instead so we don't need this extra class.
• RawMixin - Keep only the needed functions in the Problem block and remove the rest. Update any tests that need to be updated.

These are legacy implementations and add unnecessary technical debt.

We should refactor the affected XBlocks to remove any reliance on XModule-style internals and drop these mixins entirely from xblocks-contrib XBlocks.

Pre-requisite epic:

#125

[farhan]

As per discussion,

@irtazaakram will start work on this epic for Problem XBlock and do the required changes on both xblocks-contrb/Problem Block and openedx-platform . Once it's done we will do the required work for the other blocks as well.

In openedx-platform we are possibly going to attach XModuleMixin for the ContentLibrary runtime as well.