update-bash: discourage direct use

Remove the executable bit from the file to make it clear it is not
supposed to be executed directly. This should make the shebang line and
the early check also unnecessary.

Library/Homebrew/cmd/update-bash.sh

@@ -1,12 +1,3 @@
-#!/bin/bash
-
-if [[ -z "$HOMEBREW_BREW_FILE" ]]
-then
-  # we don't use odie here, because it's only available when this script is called from brew.
-  echo "Error: $(basename "$0") must be called from brew!" >&2
-  exit 1
-fi
-
 brew() {
   "$HOMEBREW_BREW_FILE" "$@"
 }