Fix env mishandling quietly killing commands

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
* sources the included `rbenv-plugin-env` instead of trying to source `rbenv-path-env` (although I’m not sure this is needed at all: as far as I can see, all this file **effectively** does is setting stuff for debug purposes, as `RBENV_ROOT` is used before it is defined there, the plugin specific variables aren’t applicable, and the `RBENV_PLUGIN_BRANCH` default is set in the individual commands – still, I’ve erred on the side of caution) and
* drops trying to source the `rbenv-path-shared` file entirely, as the functions defined there are not used anywhere in the code base.