Revert "TST: Precision issues with infer on OSX"

This reverts commit 0a5026f2ce1c61a7aceb5f22addbac4b9f8fbab1.
BUG: downcasting via com._possibly_downcast_to_dtype now rounds before casting
     to avoid precision issues on platform; dtype conversion to int is not more robust
     related GH5174
BUG/TST: transform and filter on non-unique index, closes #4620

This is a relatively minor change. Previously, filter on
SeriesGroupBy and DataFrameGroupby and transform on only
SeriesGroupBy referred to the index of each group's object.

This cannot work with repeated indexes, because some repeated
indexes occur between more than one group. Instead, use
{Series, DataFrame}GroupBy.indices array, an array of
locations, not labels.