Remove gcov-report-files-all.zip

This artifact exists on some CI builds, like
aosp_cf_x86_64_only_phone-trunk_staging-userdebug_coverage,
aosp_cf_x86_64_phone-trunk_staging-userdebug_coverage_bionic, and
cf_x86_64_phone-trunk_staging-userdebug_coverage, but it's always
an empty zip. I checked with qingshen@ and she confirmed it's deprecated
and unused. Remove it.

Also remove all usages of the *_OUT_COVERAGE variables as they were
only used for building this zip.

Bug: 413425319
Test: Presubmits
Change-Id: I0f9a5ab219720c250bbf81323e29451f3aa88bd1