Removes unused Dialyzer ignored warnings (#182)

From OTP 26, the Dialyzer warnings for Jason.Encoder's __impl__
unknown functions are not being raised anymore.

This makes the ignored warnings unnecessary, especially because CI
only runs Dialyzer with Elixir 1.15 and OTP 26.

Co-authored-by: antedeguemon <[email protected]>

Author: antedeguemon

dialyzer.ignore

@@ -44,7 +44,6 @@ defmodule Jason.Mixfile do
 
   defp dialyzer() do
     [
-      ignore_warnings: "dialyzer.ignore",
       plt_add_apps: [:decimal]
     ]
   end