refactor(scan): remove yum-security related code

[kotakanbe]

What did you implement:

The yum-plugin-security related code was left for amazon Linux but is no longer needed.
Because Amazon OVAL scanning has been supported in #821 .
To scan Amazon Linux, You have to fetch amazon's OVAL before reporting.
see https://github.com/kotakanbe/goval-dictionary#usage-fetch-amazon-alas-as-oval-data-type

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce.

Checklist:

You don't have to satisfy all of the following.

• Write tests
• Write documentation
• Check that there aren't other open pull requests for the same issue/feature
• Format your source code by make fmt
• Pass the test by make test
• Provide verification config / commands
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: YES