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ARROW-8619: [C++] Use distinct enum values for MonthInterval, DayTimeInterval

This enables us to eliminate "special" handling of `Type::INTERVAL` since these types have a different internal data representation. The deleted code in this PR is evidence of this.

This is a breaking API change but since this is a lesser-used part of the project I don't think that many users will be impacted.

Closes #7060 from wesm/ARROW-8619

Authored-by: Wes McKinney <wesm+git@apache.org>
Signed-off-by: Wes McKinney <wesm+git@apache.org>