Fix env mishandling quietly killing commands [take 2]

Several *rbenv-plugin* commands try to source `rbenv-path-env` and `rbenv-path-shared`, which will fail as both files are not present in the plugin `bin` directory. With *rbenv* setting `set -e`, this quietly kills the command without feedback as to what happened.

This PR removes all dependencies on `rbenv-path` by
* sourcing the included `rbenv-plugin-env` instead of trying to source `rbenv-path-env`,
* removing the `rbenv-path` guard from `rbenv-plugin-env` and
* removing the `rbenv-*-shared` sourcing entirely, as the functions defined there are not used anyway.