Moves Codecov Ignore to LCOV

Moves Codecov Ignore to LCOV

Rely on ax_code_coverage to exclude test directories.

- Removes broken codecov ignore
- Places ignore section in ax_code_coverage
- Forwards users from codecov to LCOV for ignores

Reviewed-by: Prakash Surya <prakash.surya@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Signed-off-by: Kjeld Schouten-Lebbing <kjeld@schouten-lebbing.nl>
Closes #9726