Change of env var restoration mechanism in activate script

With the current system, kerl assumes that nothing else may touch your `$PATH` and `$MANPATH` once it has been activated. However, this is not necessarily the case.
Instead of blindly backing up from a potentially stale copy, this PR proposes a change to more direct removal of the  addition from `$PATH` and `$MANPATH` which is known at the time of defining `kerl_deactivate`