Remove non-essential XSLT from Schematron (source: 18F/fedramp-automation: 503)

[danielnaab]

Original issue: https://github.com/18F/fedramp-automation/issues/503

Extended Description

• As a 10x ASAP developer, in order to provide clean Schematron code, I want all non-essential XSLT constructs removed from ssp.sch.

Acceptance Criteria

• [x] All Schematron assertion messages are declarative statements which affirm the positive test outcome.
• [x] All Schematron assertion diagnostic messages are declarative statements which explain the negative test outcome.
• [x] The Schematron code has no assertion failures when validated using src/validations/styleguides/sch.sch using the basic phase.
• [ ] XSpec unit tests for positive and negative Schematron assertion outcomes accompany all Schematron assertions (where feasible).
• [x] Non-essential XSLT code is refactored into pure Schematron.
• [x] Essential XSLT code is minimized and highlighted within the code as anomalous.

Story Tasks

• [x] Identify non-essential XSLT code.
• [x] Refactor non-essential XSLT code.
• [x] Minimize and highlight essential XSLT code.
• [ ] Test transpiled Schematron.
• [ ] Profile transpiled Schematron.

Definition of Done

• [ ] Acceptance criteria met
• [ ] Unit test coverage of our code > 95%
• [ ] Automated code quality checks passed
• [ ] Security reviewed and reported
• [ ] Reviewed against plain language guidelines
• [ ] Code must be self-documenting
• [ ] No local tech debt
• [ ] Load/performance tests passed – needs to be created/automated
• [ ] Documentation updated
• [ ] Architectural Decision Record completed as necessary for significant design choices
• [ ] PR reviewed & approved
• [ ] Source code merged