Remove obsolete cbmc/bmc_cover [blocks: #4217]

[peterschrammel]

Replaced by goto-checker infrastructure

Based on #4215 - last commit

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[tautschnig]

@peterschrammel clang-format isn't entirely happy just yet.

[allredj]

⚠️
This PR failed Diffblue compatibility checks (cbmc commit: 55f7baf).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/103099525
Status will be re-evaluated on next push.
Common spurious failures:

• the cbmc commit has disappeared in the mean time (e.g. in a force-push)

• the author is not in the list of contributors (e.g. first-time contributors).

• the compatibility was already broken by an earlier merge.