remove cssmin dependency

[sssoleileraaa]

Status

Ready for review

/cc @rmol

Test Plan

• make clean
• make build-debs
• make staging

• Verify that CSS is delivered from staging instances by checking that the UI looks as expected
• Verify that CSS is minified by Sass by comparing file sizes

Description of Changes

Fixes #2808

We can remove our cssmin dependency because we already use a ruby script that we tell to compress our css files at debian packaging time.

Changes proposed in this pull request:

• Remove cssmin dependency from requirements.txt and requirements.in

Checklist

If you made changes to the server application code:

• Linting (make ci-lint) and tests (make -C securedrop test) pass in the development container

If you made changes to securedrop-admin:

• Linting and tests (make -C admin test) pass in the admin development container

If you made changes to the system configuration:

• Configuration tests pass

If you made non-trivial code changes:

• I have written a test plan and validated it for this PR

If you made changes to documentation:

• Doc linting (make docs-lint) passed locally