Erlang - remove reference to old API fn

This is no longer present in sp_midi

Author: samaaron

app/server/erlang/sonic_pi_server/src/sp_midi/sp_midi.erl

@@ -1,6 +1,6 @@
 -module(sp_midi).
 -export([midi_init/0, midi_deinit/0, midi_send/2, midi_flush/0, midi_ins/0, midi_outs/0, have_my_pid/0,
-        set_this_pid/1, set_log_level/1, schedule_callback/3, get_current_time_microseconds/0]).
+        set_this_pid/1, set_log_level/1, get_current_time_microseconds/0]).
 -on_load(init/0).
 
 -define(APPLICATION, sonic_pi_server).
@@ -40,5 +40,3 @@ set_log_level(_) ->
     exit(nif_library_not_loaded).
 set_this_pid(_) ->
     exit(nif_library_not_loaded).
-schedule_callback(_, _, _) ->
-    exit(nif_library_not_loaded).