Remove application callback and persistent_term check

persistent_term has been available since OTP 21.2 and Mint requires
Elixir ~> 1.12 which needs OTP 22+, so it is always available. Remove
the runtime check, the application env, and the empty supervisor.

Author: ericmj

lib/mint/application.ex

@@ -601,18 +601,14 @@ defmodule Mint.Core.Transport.SSL do
   end
 
   defp get_cacertfile(path) do
-    if Application.get_env(:mint, :persistent_term) do
-      case :persistent_term.get({:mint, {:cacertfile, path}}, :error) do
-        {:ok, cacerts} ->
-          cacerts
-
-        :error ->
-          cacerts = decode_cacertfile(path)
-          :persistent_term.put({:mint, {:cacertfile, path}}, {:ok, cacerts})
-          cacerts
-      end
-    else
-      decode_cacertfile(path)
+    case :persistent_term.get({:mint, {:cacertfile, path}}, :error) do
+      {:ok, cacerts} ->
+        cacerts
+
+      :error ->
+        cacerts = decode_cacertfile(path)
+        :persistent_term.put({:mint, {:cacertfile, path}}, {:ok, cacerts})
+        cacerts
     end
   end
 

lib/mint/core/transport/ssl.ex

@@ -16,7 +16,6 @@ defmodule Mint.MixProject do
       # Xref
       xref: [
         exclude: [
-          :persistent_term,
           {:ssl, :cipher_suites, 1},
           {:public_key, :cacerts_get, 0},
           CAStore
@@ -54,8 +53,7 @@ defmodule Mint.MixProject do
   # Run "mix help compile.app" to learn about applications.
   def application do
     [
-      extra_applications: [:logger, :ssl],
-      mod: {Mint.Application, []}
+      extra_applications: [:logger, :ssl]
     ]
   end