Disable calling --slice-formula by default with conditional flags.

[jaisnan]

Description of changes:

Hot fix for disabling CBMC's --slice-formula. --slice-formula was being called by default which led to some unnecessary values being presented in the trace. This PR disables the CBMC flag when calling --visualize or when we run Kani with --no-slice-formula.

Resolved issues:

Resolves #1288

Call-outs:

Since the fix is a just conditionally enabling a flag, no tests are necessary imo. Calling Cargo Kani with --dry-run reveals that --slice-formula is disabled when calling --visualize or --no-slice-formula.

A test would be more expensive than is needed.

Testing:

• How is this change tested?
  Checked with dry-run if the extra flag is still being called.

• Is this a refactor change?
  Yes

Checklist

• Each commit message has a non-empty body, explaining why the change was made
• Methods or procedures are documented
• Regression or unit tests are included, or existing tests cover the modified code
• My PR is restricted to a single feature or bugfix

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.

[tedinski]

If you fix the minor formatting error quickly, we can probably get this in before release today.