`videosequence` and `problemset` have been replaced with `sequential`.
`problemset` and `videosequence` are old-but-not-entirely-unused aliases to the `sequential` block type (in Studio-speak, "Subsection").
Since [these block types have been removed from the 6 courses that used them](https://openedx.atlassian.net/browse/DEPR-151?focusedCommentId=588197), this ticket removes the support for the `problemset` and `videosequence` block-types.
For more information, see ticket: [DEPR-151](https://openedx.atlassian.net/browse/DEPR-151)
By transparently proxying between the XModuleDescriptor and the XModule,
and between their runtimes, we can make them act as a single class, so
that we can swap in an actual XBlock instead.
