[BUGFIX] Remove zsh-update

Remove unncessary file.
[FEATURE] Enable history-substring-search in zsh.

Manually source history-substring-search.zsh in .zshrc. Had to source manually
because adding plugin to oh-my-zsh did nothing. Apparently this is a known bug
that affect Debian-based distros?

https://github.com/robbyrussell/oh-my-zsh/issues/1433
https://github.com/robbyrussell/oh-my-zsh/issues/2664