Remove _tarsnap

[syohex]

because it provides the zsh completion file now

• [ ] This compdef is not already available in zsh.
• [ ] This compdef is not already available in their original project.
• [ ] I am the original author, or I have authorization to submit this work.
• [x] This is a finished work.
• [ ] It has a header containing authors, status and origin of the script.
• [ ] It has a license header or I accept that it will be licensed under the terms of the Zsh license.

See #845