[MLIR] Remove leftover field from Variadic, NFC (#176373)
There appears to be an unused field in the `Variadic` TableGen class,
introduced originally in commit
70b69c54fa8b24519cd549eec10e549471157bb8. It looks like, when removing
the associated `VariadicAtLeast` class
(bf8049dc483131365aea3d3626a2d27b5dfa92de), this field still remained.
