test: Remove #! line from test-lib.sh

It makes no sense to run test-lib.sh, so it makes no sense to give it
an interpreter.  This is particularly annoying for Emacs users who
have executable-insert set, since the presence of the #! line will
cause Emacs to mark test-lib.sh executable when saving it, which will
in turn case the 'basic' test to fail.

test/test-lib.sh

@@ -1,4 +1,3 @@
-#!/usr/bin/env bash
 #
 # Copyright (c) 2005 Junio C Hamano
 #