fly: remove writeText

The writeText argument is unused. It was originally used to copy a bash
completion script into the derivation, but a different approach was
chosen, and I simply forgot to remove the writeText argument.

78013b4ecc
https://github.com/NixOS/nixpkgs/pull/64714#discussion_r303216977

pkgs/development/tools/continuous-integration/fly/default.nix

@@ -1,4 +1,4 @@
-{ buildGoModule, fetchFromGitHub, stdenv, lib, writeText }:
+{ buildGoModule, fetchFromGitHub, stdenv, lib }:
 
 buildGoModule rec {
   pname = "fly";