[mlir][ods] Remove warning in `AttrOrTypeDef`

This warning was added because using attribute or type assembly formats
with `skipDefaultBuilders` set could cause compilation errors, since the
required builder prototype may not necessarily be generated and would
need to be checked by hand. This patch removes the warning because a
warning that the generated C++ "might" not compile is not particularly
useful. Attempting to address the TODO (i.e. detect whether a builder of
the correct prototype is provided) would be fragile since it would not
be possible to account for implicit conversions, etc.

In general, ODS should not be emitting warnings in cases like these.

Reviewed By: rriddle, wrengr

Differential Revision: https://reviews.llvm.org/D130102