Remove old/unused dependencies

[jmaupetit]

Purpose

The following dependencies are no longer used in production:

• password-container-xblock
• xblockutils2

We need to remove them.

Proposal

• Add a migration to remove underlying table & entries
• Remove them from the INSTALLED_APPS in LMS & CMS settings

[rmoch]

xblockutils2is still used by proctoru-xblock