Remove shebangs from completions (#980)

Shebangs are only used for scripts that are intended to be executed, but
completions are meant to be sourced by the shell, and so shebangs are
not required.

I've also renamed the completions so that editors can continue to detect
the correct file format.

completions/fastfetch.bash

@@ -1,5 +1,3 @@
-#!/usr/bin/env bash
-
 __fastfetch_complete_help()
 {
     local __ff_helps=(

completions/fastfetch.fish

@@ -1,5 +1,3 @@
-#!/usr/bin/env fish
-
 if not type -q fastfetch
     exit
 end