removes old unused shared/errors/_backtrace partial

test plan:
n/a, removing old unused code

Change-Id: I7cfd1653e51121dcdca9a85f5951c55d43928c92
Reviewed-on: https://gerrit.instructure.com/29107
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jon Jensen <jon@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Marc LeGendre <marc@instructure.com>