Revert "Allow installation of bundler even when puppet is within rbenv"

This fix was not necessary because `rbenv exec` prepends an entry to the
path[1] that will take precedence ofer the entry that's already exists
there.

Additionally, passing `PATH` to an `exec` resource like this triggers
puppet warnings, which leads to lots of noise in puppet runs.

This reverts commit 5a542313e956a48ef708cae7ae54d005a0fb66a1.

[1] https://github.com/sstephenson/rbenv/blob/master/libexec/rbenv-exec#L45