Remove unneeded function

It's only used in one place, no need for the abstraction currently.

lib/display/failure.ex

@@ -10,7 +10,10 @@ defmodule Display.Failure do
     """
   end
   def format_failure(%{error: %ExUnit.AssertionError{expr: expr}, file: file, line: line}) do
-    format_assertion_error(expr, file, line)
+    """
+    #{Paint.cyan("Assertion failed in #{file}:#{line}")}
+    #{Paint.red(Macro.to_string(expr))}
+    """
   end
   def format_failure(%{error: error, file: file, line: line}) do
     """
@@ -19,13 +22,6 @@ defmodule Display.Failure do
     """
   end
 
-  defp format_assertion_error(error, file, line) do
-    """
-    #{Paint.cyan("Assertion failed in #{file}:#{line}")}
-    #{Paint.red(Macro.to_string(error))}
-    """
-  end
-
   defp format_error(error) do
     trace = System.stacktrace |> Enum.take(2)
     Paint.red(Exception.format(:error, error, trace))