Both the `lcov_merger` and the `collect_cc_coverage` script are obtained from "well-known" implicit attributes of Starlark rules and do not require any further special handling.

Removing this special handling also fixes a fringe bug: When a `cc_test` is wrapped in another test target that applies a transition to it, the removed logic set the `LCOV_MERGER` variable to the untransitioned path of the `lcov_merger`, which will then not be available in the test sandbox since it is only added as a runfile, but the test action references it via its exec path. This resulted in a "file not found" error that broke coverage.

Closes bazelbuild#20349.

PiperOrigin-RevId: 589252093
Change-Id: I810f1b414dcfedcc8930a74390ef5c9bfd2cd030