[mlir] Remove the `type` keyword from type alias definitions

This was carry over from LLVM IR where the alias definition can
be ambiguous, but MLIR type aliases have no such problems.
Having the `type` keyword is superfluous and doesn't add anything.
This commit drops it, which also nicely aligns with the syntax for
attribute aliases (which doesn't have a keyword).

Differential Revision: https://reviews.llvm.org/D125501