CLN: Remove unnecessary ExcelWriterMeta metaclass

Only `__new__` is really necessary. And subclasses just have to deal with
being passed an 'engine' keyword argument + a tad bit of style cleanup.

Thanks to @jseabold and @josef-pkt for making me realize that this was
unnecessary in https://github.com/statsmodels/statsmodels/issues/1167
:)

@jmcnamara - this has completely trivial impact on ExcelWriter
subclasses, just removes some magic.