[scripts] Clean up unnecessary steps in coverage_report.sh
[scripts] Clean up unnecessary steps in coverage_report.sh

There should never be a need to remove gcda and gcno files
from the target directory:
- The script first runs "cargo clean", which removes the old target
  directory.
- Running "cargo build" will not create any data (gcda) files.
- Any gcno files created during the build should be kept, except...
There is no need to first run "cargo build" since "cargo test" will
also build the files to be tested.

Closes: #977
Approved by: darioncassel

aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,libra/libra,libra/libra,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core