`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)
A "section" tag in an OLX upload used to map to the
SemanticSectionDescriptor, which translated it into a Sequence
("sequential" tag). This is both obscure and confusing, since it uses
language that predates Studio. Back in the LMS prototype days,
"section" was inconsistently used to be interchangeable with "sequence"
and "sequential", and what Studio today calls a "section" was called a
"chapter". Bits of this legacy terminology are still around in the
courseware rendering code.
The upshot is that if you make an OLX tag "section" before this commit,
it would not map to what we call a "Section" in all our documentation,
but to a "Subsection"; furthermore, if that <section> only had one child
element, the node would be replaced with its singular child, removing
the <section> node from the course tree entirely.
The fact that you can make a "section" OLX tag
at all is nowhere in our documentation because courses haven't been
written that way since late 2011 or early 2012.
SemanticSectionDescriptor came up as part of the XModule ->
XBlock conversion efforts as a legacy XModule that isn't worth
converting. With the removal of this class, all XBlocks
in edx-platform are "pure" XBlocks, ending our reliance
on the XModule-to-XBlock shimming infrastructure.
This commit also removes the process_includes decorator, which was only
used for "section" tags. This does NOT delete the ProblemBlock-specific
<include> tag, which is still supported (if obscure).
There is a chance that through tribal knowledge or copy-paste, some
section tags survive in the wild of old edX courses. It's difficult for
us to assess because by its nature, this tag doesn't just say
"section", but instead actually does the mutation on import so it's
stored as "sequential" in the modulestore–therefore things like
CourseGraph can't detect it.
The fix for any such XML-authored courses is:
* For instances of <section> that wrap a single child node,
replace the <section> node in favor of its child node.
* For instances of <section> that wrap a sequence of children,
substitute <section> with <sequential>
Note that "<section>" is a valid HTML tag
type and so may show up in any component that can contain HTML and is
unrelated to the course structure OLX tag alias "<section>" that this
commit removes.
DEPR-124
