Remove forgotten Pleroma.OTPVersion usage in mix.exs

This was used in OTP releases where the normal OTP_VERSION file
is unavailable.

If checking against OTP minor versions and patch levels
is needed again, revert this commit and commit mentioned below.

Context: 1be8deda73

mix.exs

@@ -37,22 +37,13 @@ defmodule Pleroma.Mixfile do
         pleroma: [
           include_executables_for: [:unix],
           applications: [ex_syslogger: :load, syslog: :load, eldap: :transient],
-          steps: [:assemble, &put_otp_version/1, &copy_files/1, &copy_nginx_config/1],
+          steps: [:assemble, &copy_files/1, &copy_nginx_config/1],
           config_providers: [{Pleroma.Config.ReleaseRuntimeProvider, []}]
         ]
       ]
     ]
   end
 
-  def put_otp_version(%{path: target_path} = release) do
-    File.write!(
-      Path.join([target_path, "OTP_VERSION"]),
-      Pleroma.OTPVersion.version()
-    )
-
-    release
-  end
-
   def copy_files(%{path: target_path} = release) do
     File.cp_r!("./rel/files", target_path)
     release