Remove unused "between-committees.jinja" template

[jenniferthibault]

Discovered through https://github.com/18F/fec-cms/issues/1605

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Between committees used to be a tab under the Committee profile pages, but since the data was self-reported it introduced weirdness that made it seem like it was not accurate enough to display ( fecgov/openFEC-web-app#1448 )

In fecgov/openFEC-web-app#1819 and fecgov/openFEC-web-app#2019 it looks like that information was gathered under the Spending tab to make consistent models across the site, and reconfigured into Disbursement transactions, so we no longer use or need the .jinja template.

If we return to it in the future, we can get it back through GitHub's commit history

Completion criteria:

• Unused template fec/data/templates/partials/committee/between-committees.jinja is deleted from the repository

[jenniferthibault]

Moved to "Done" since it's part of https://github.com/18F/fec-cms/pull/1641 awaiting review.