Remove g_poa_effective renamed kwarg warning in pvsystem.pvwatts_dc()

[RDaxini]

• Closes Remove g_poa_effective renamed kwarg warning in pvsystem.pvwatts_dc() #2703
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Also removes variable from the nomenclature page

[RDaxini]

Tagged as 0.15.1 but the wrapper did allow users to use the old arg name, as well as emit a warning, so do we class it as a breaking change for 0.16.0...?

[AdamRJensen]

Tagged as 0.15.1 but the wrapper did allow users to use the old arg name, as well as emit a warning, so do we class it as a breaking change for 0.16.0...?

I would say yes, this should go in 0.16.

[AdamRJensen]

@RDaxini Could you also remove the term from the nomenclature? It's a bit of a scope creep, but I think it fits nicely here 😄