Removed overlapping_spans.{rs,stderr,nll.stderr}.

This is based on the feedback from estebank:

"""
I believe that test can be removed outright. It'd be impossible for a
new change to go through that breaks this kind of output without it
being picked up by multiple other `stderr` tests. This is an artifact
of the transition period to the "new" output style.
"""

see: rust-lang#52663 (comment)

Author: pnkfelix