builder-run: Drop unused script

This script is from f7103ba (build: add back builder-run utility
script, 2017-05-09, coreos/tectonic-installer#609), but doesn't seem
to have ever been used anywhere.  With installer/scripts gone since
7296010 (frontend: Remove frontend code and backend API code,
2018-04-06, coreos/tectonic-installer#3067), I think we can just drop
builder-run entirely.

Author: wking