Remove t() helper override

All spree core codebase now uses Spree.t() so I think we can safely
remove it. Plus I don't believe it's a good idea to throw warning every
time t() is called. Spree extensions or even other libraries can just
choose not to use Spree.t() at all. I don't think we should force then
to.

Guys do you think we still need that method?