TEST_MAPPING: finish renaming the test suite

Now that we've created a copy of the existing test suite with the intended
name, and run it through postsubmit for SLO measurement, we can finally move it
to presubmit, and delete the old duplicate suite.

Test: cd external/llvm-libc && atest
Link: go/test-mapping-slo-guide
Change-Id: I07026ec24f86788f6fad2069a6eccbc0a0d6ef0f