Revert^2 "Turn uses of `COVERAGE_PATHS` and `COVERAGE_EXCLUDE_PATHS` into errors."

All known uses of environment variables `COVERAGE_PATHS` and
`COVERAGE_EXCLUDE_PATHS` (in AOSP and downstream branches) have
been replaced with `NATIVE_COVERAGE_PATHS` and
`NATIVE_COVERAGE_EXCLUDE_PATHS` (resp.).

This reverts commit 62d99126.

Reason for revert: The cause of the build breakages caused by the
initial change has been addressed.

Test: m nothing
Test: Check that `m nothing COVERAGE_PATHS=art` fails.
Test: Check that `m nothing COVERAGE_EXCLUDE_PATHS=art` fails.
Test: Check the Android Build for build errors related to these variables.
Bug: 158212027
Bug: 160769227
Change-Id: Ic267e1b61e84bc6f016a3eb57efe7f9b859adb42