Remove duplicated documentation [blocks: #3258]

[johnnonweiler]

This PR reverts #3244.

#3244 added two copies of the CPROVER manual - assumptions.md is a exact copy of doc/cbmc-user-manual.md which already existed in the repository, and the other files contain another copy of the user manual with some changes.

The new version (which doesn't currently work, and is removed by this PR) is: http://cprover.diffblue.com/md__home_travis_build_diffblue_cbmc_doc_cprover-manual_index.html

The existing version (which this PR keeps) is: http://cprover.diffblue.com/cbmc-user-manual.html

For information, the changes I can see in the new version were:

• Breaking up the documentation into multiple files
• A new index to link to those files, instead of using the index automatically generated by Doxygen as in the current version
• Links using a different link format which is incompatible with Doxygen
• Removing some parts, including (for example) the sections about hardware/software co-verification
• Slightly reordering the remaining sections
• Replacing three backticks with three backticks followed by the letter C (to get C syntax highlighting)
• (Other minor changes, e.g. changing section titles)

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: b37f1de).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/90398675

[johnnonweiler]

I'm told the intention is to remove the existing version, and use the new version. I've created #3295 to fix the current problem with Doxygen, and hope that soon we can follow-up with another PR to remove the duplicates.