CLN: Remove unused and undocumented kind keyword from read_excel
Pops kwarg and warns if you pass it (rather than passing on).

Fixes #4712

maybe kind was meant to be engine/flavor  at some point to support different backends
 I think it might have been for xlsx/xls.

For now, it's just weird to have something in the function signature that's
completely unused, so better to remove it (but, for now, won't error out or
anything if you pass it).
 @jreback and others - any objections to merging this?
 looks good
