Silently Ignored Kwargs Cleanup
Inspired by #18699 I wrote up a very brief script that when placed in the root directory could help identify functions where ``**kwargs`` are accepted but not used in the method body. While not perfect (may give false positives in cases of nested function definitions) it calls out 128 functions as is (see [extra_kwargs.txt](https://github.com/pandas-dev/pandas/files/1553193/extra_kwargs.txt)). A quick spot check on my end looked good.

Sharing in case anyone would like to look through this and submit PR's to tackle in batches. Here's the script I used in case anyone is interested

    import re
    import glob

    if __name__=='__main__':
        pater = re.compile(r"""
        ^(.+?)       # group to match function name (non-greedy)
        \((.*?)\)    # group to match arguments
        (.*)$        # everything else (function body)
        """, flags=re.DOTALL|re.VERBOSE)

        fns = glob.glob('pandas/**/*.py', recursive=True)
        for fn in fns:
            with open(fn) as infile:
                source = infile.read()
                funcs = source.split('def ')
                for func in funcs:
                    res = re.search(pater, func)
                    if res:
                        nm, args, body = res.groups()
                        if '**kwargs' in args and not 'kwargs' in body:
                            args = re.sub('\s+', ' ', args)
                            print("{} def {}({})".format(fn, nm, " ".join(
                                args.split("\n"))))