Remove redundant and unreliable coverage test results

The `coverage-reports` tests still generate counters and JSON reports
for inspection, but these files are no longer used in Makefile diffs, to
reduce complexity and confusion from unreliable or unexpected test
results, especially when maintaining them (i.e., generating `--bless`ed
results).

The associated `expected_` files for counters and JSON reports have been
removed, leaving only the files actually used for testing: the `llvm-cov
show` reports.

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