Remove reference to deleted `repository.litani_root()` from `setup.py`

[hanno-becker]

If litani is not in the PATH, cbmc-starter-kit-setup fails because of the reference https://github.com/model-checking/cbmc-starter-kit/blob/master/src/cbmc_starter_kit/setup.py#L80 to the no-longer existing repository.litani_root().

From https://github.com/model-checking/cbmc-starter-kit/commit/d7aa4eed99d2dfe8fbef362396746451b48ce23c it seems that the branch should be removed: "Developers, who contribute to projects using the cbmc-starter-kit, now add Litani to their system's PATH and no longer add it to their project as a submodule."

Also, should litani be a dependency of cbmc-starter-kit in brew?