[mlir][tblgen] Reverting fatality of assemblyFormat with skipDefaultBuilders=1

Per @rriddle, we do not want to require `skipDefaultBuilders=0` per se; that is, even though the `assemblyFormat`-generated parser requires a builder with the same prototype as the default-builder, that prototype could instead be implemented via custom `builders`.  This differential reduces the FatalError introduced in D128555 to a non-fatal Warning instead, so that users can still be informed of the error condition (rather than waiting for the C++ compiler to fail).

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D129234