Revert "Merge branch 'extend-default-cmake-prefix-path-of-slot' into 'master'"

This reverts merge request !302 (merged)

The problem with the change is that it extends the CMAKE_PREFIX_PATH before it is modified by the toolchain, which means that the LCG packages will still be preferred over those built in the slot (unless there are other reasons to prefer our build, like an explicit constraint on the version).

See LBCORE-1906