[RFC] Remove SATABS documentation

[tautschnig]

SATABS is not (currently) part of the source tree, and only available in binary format via https://www.cprover.org/satabs/ (the source URL is 404). Unless we choose to import the source into this repository we should likely remove doc/satabs-user-manual.md from the the source tree.

[martin-cs]

Either remove the documentation or put the source on github. There is no point having documentation for a tool that people can't really use any more.