[mlir][python] Don't emit diagnostics when printing invalid ops

The asm printer grew the ability to automatically fall back to the
generic format for invalid ops, so this logic doesn't need to be in the
bindings anymore. The printer already handles supressing diagnostics
that get emitted while checking if the op is valid.

Reviewed By: mehdi_amini, stellaraccident

Differential Revision: https://reviews.llvm.org/D144805