Remove `mod_h26x`

[JessamyT]

Fixes #218, also related to #204 and #220 but does not close them.

Needs review for accuracy of what video codecs are actually still supported. h26x, theora, and mp4 seem to be deprecated but not sure what is now built-in versus unsupported versus supported by other modules. Went based on a couple of comments but didn't find reference to VP9 elsewhere so that should perhaps come out.

Module file seems to already be deleted