Update rbenv module

[fantasyzhjk]

heres the changes

• Removed outdated code
• Replaced the old load-env with $env assignment
• Kept the rbenv command intact but focused on the "rbenv shell" (modified a subcommand rather than the whole command)
• Deleted unused command completions

[fdncred]

a description of what you changed and why you changed it would be helpful

[fdncred]

Thanks