Remove references to qa-sast and fix mitmproxy install

What does this MR do?

• Fixes broken mitmproxy install by switching from debian package version to pip3 version

  Package mitmproxy is not available, but is referred to by another package.
  This may mean that the package is missing, has been obsoleted, or
  is only available from another source
  E: Package 'mitmproxy' has no installation candidate

• Remove references to qa-sast because SAST no longer uses downstream QA tests as of Disable SAST and SD downstream QA tests (gitlab-org/security-products/ci-templates!356 - merged) • James Liu • 16.1, and we're now encountering failing pipelines because qa-sast.yml has been completely removed in Fix verify_templates script (gitlab-org/security-products/ci-templates!474 - merged) • Adam Cohen • 18.4.

What are the relevant issue numbers?

• Remove all SAST QA projects and references (gitlab-org/gitlab#567203 - closed) • Adam Cohen • 18.4
• Improve SAST analyzer tests (gitlab-org/gitlab#366852 - closed) • rossfuhrman, Lucas Charles+ • 15.9 • Needs attention
• gemnasium-python-dependency_scanning job is fai... (gitlab-org/gitlab#567848 - closed) • Unassigned • 18.4