ZSH and Bash completion: remove shebang (#1248)

Shebangs are only for files that are directly executable. In cases
where files are only sourced (such as completion scripts), these
are unneeded.

shell/completion.bash

@@ -1,4 +1,3 @@
-#!/bin/bash
 #     ____      ____
 #    / __/___  / __/
 #   / /_/_  / / /_

shell/completion.zsh

@@ -1,4 +1,3 @@
-#!/bin/zsh
 #     ____      ____
 #    / __/___  / __/
 #   / /_/_  / / /_