Remove Grob_info::origin_contexts ()

Remove Grob_info::origin_contexts ()

I cannot believe this: this function is obscure both in definition and
usage.  Most uses were completely nonsensical, and one use just an
overly complex method of achieving a semi-random goal.

I cannot figure out what the original aim of the respective code
passages was, but whatever it was, seemingly other workarounds took
its place successfully.


Also contains some preparatory commits:

Remove Grob_info::origin_contexts ().size () uses

The size of Grob_info::origin_contexts () was used several times as a flag,
but the function could never return an empty array anyway.


Sanitize Break_align_engraver::create_alignment source

Break_align_engraver went to some contortions to get an engraver from
the same context as the grob it acknowledged.  There is no real point
in it not just using the origin engraver for that purpose, however.

[Dan Eble, 2018-07-07 21:16:58]

LGTM