CLN/PERF: simplify tslib.get_time_micros 
`tslib.get_time_micros` implementation is way more complicated than it needs to be.  Instead of calling `dt64_to_dtstruct` in a loop, this changes the function to one call to `np.mod`.

AFAIK the `np.mod` call is to the python function, not sure if numpy exposes a cython version of mod.   Then again, the function is only used once in `indexes.datetimes` and isn't all that perf sensitive.  It might not need to be in cython in the first place.

Re-wrote inaccurate docstring.