Remove __unittest_toString and associated version(druntime_unittest).

This was introduced as part of GitHub pull request #41, which
was questionable (!) support code for D-Programming-Language/dmd#263.
However, the latter never materialized and __unittest_toString plus
version(druntime_unittest) were just confusing oddities at this point.
Merge pull request #856 from klickverbot/remove-dead-tostring

Remove __unittest_toString and associated version(druntime_unittest).