Deprecate `Formula#needs` and remove `needs :openmp` support

The only remaining use case for `needs` DSL was `needs :openmp`, but
this was somewhat deprecated via an audit on 2017-12-30 in
f9b6407110d7fb98a65b470d166fc5b7be6052ee. This makes the deprecation
official via `odeprecated` so we can remove the unused code in future
brew release