Delete some compdefs that have been included in zsh (thanks @okapia)

Command used, on a zsh 5.6.2 installation:

    for def in src/*; { [[ -n $(find /usr/share/zsh/functions -name ${def:t}) ]] && git rm $def }

Closes zsh-users#607

CONTRIBUTING.md

@@ -8,7 +8,7 @@ Contributions are welcome, just make sure you follow the guidelines:
  * Completions are not accepted when already available in their original project.
  * Please do not just copy/paste someone else's completion, ask before.
  * Partially implemented completions are not accepted.
- * Please add a header containing authors, status and origin of the script and license header if you do not wish to use the Zsh license (example [here](src/_ack)).
+ * Please add a header containing authors, status and origin of the script and license header if you do not wish to use the Zsh license (example [here](src/_tox)).
  * Any reasonable open source licence is acceptable but note that we recommend the use of the Zsh license and that you should use it if you hope for the function to migrate to zsh itself.
  * Please try to follow the [Zsh completion style guide](https://github.com/zsh-users/zsh/blob/master/Etc/completion-style-guide).
  * Please send one separate pull request per file.