Remove usage of lower_extensions

[diptorupd]

Numba 0.57 removed the use of lower_extension for parfor and converted the parfor passes into regular Numba passes rather than passes that are registered using an extension.

Numba-dpex still uses the old lower_extension feature in its lowering passes to register a custom parfor lowering pass. The design needs updating to port numba-dpex to Numba 0.57