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Fixes #5623.

Removing as this is now installed as part of homebrew automatically and this
version is out of date. See
https://github.com/Homebrew/brew/blob/master/share/zsh/site-functions/_brew

Updated _brew to match latest in https://github.com/Homebrew/homebrew/blob/master/Library/Contributions/brew_zsh_completion.zsh

`brew services` and `brew server` were moved to homebrew-boneyard.

Add `brew cleanup` command to `bubu` alias

sorin-ionescu had some good points.

Merge completion with official brew completion. Add a helper to link official
completion into oh-my-zsh plugin (without overwriting). Add an alias to list
installed brews. Add brews to the path (in a somewhat strange way).

Reorganizing plugins so that each plugin has it's own directory now so that any plugin-specific functions can be bundled within there.