Remove `g_poa_effective` renamed kwarg warning in `pvsystem.pvwatts_dc()`

[RDaxini]

Is your feature request related to a problem? Please describe.
g_poa_effective effective was renamed to effective_irradiance in pvwatts_dc() around 8 months ago in v0.13.0

Describe the solution you'd like
Almost 1 year and 2 major releases later, I think we can remove the warning.

Describe alternatives you've considered
More time for users to be warned of the renaming.

Additional context

[cwhanse]

I agree with removing the warning in the next release