[mlir] Remove LLVMTypeTestDialect

[mlir] Remove LLVMTypeTestDialect

This dialect was introduced during the bring-up of the new LLVM dialect type
system for testing purposes. The main LLVM dialect now uses the new type system
and the test dialect is no longer necessary, so remove it.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D85224