Simplify or remove `assert_*_equal` wrappers around `pandas.testing` functions

[kandersolar]

We have our own wrappers in conftest.py for the three pandas assert_X_equal functions. We originally added those because the default tolerances of those functions changed in pandas 1.1.0 (see #1018). Our wrappers check the version of pandas and do some simple kwarg manipulation based on whether the installed pandas is greater or lower than 1.1.0.

Now that our minimum pandas is greater than 1.1.0, the version checking is no longer needed and we can simplify those functions.

It is also worth looking into whether we can/should get rid of those wrappers entirely and just use the pandas functions directly, but that change might require a lot of work (https://github.com/pvlib/pvlib-python/issues/1018#issuecomment-668631386).

[AdamRJensen]

I think we should just get rid of our own wrappers, and use pandas.