Remove unnecessary test-related code from WebUIMochaBrowserTest

The DisableSubTestResultReporting call in the test itself is redundant
now that it's being called in the test fixture class ctor. Removing this
call was missed.

SubTestReporterImpl is unused and in fact unnecessary because the
interface class itself already contains the production implementation.

Bug: 393139276
Change-Id: Id8109ae0ea973e2b7d9433841deed85d3ea8fa4d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6435372
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Alex Yang <aycyang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1443661}