py/runtime: Remove nlr protection when calling __next__ in mp_resume.

And remove related comment about needing such protection when calling send.

Reasoning for removal is as follows:
- mp_resume is only called by the VM in YIELD_FROM opcode
- if send_value != MP_OBJ_NULL then throw_value == MP_OBJ_NULL
- so if __next__ or send are called then throw_value == MP_OBJ_NULL
- if __next__ or send raise an exception without nlr protection then the
  exception will be handled by the global exception handler of the VM
- this handler already has code to handle exceptions raised in YIELD_FROM,
  including correct handling of StopIteration
- this handler doesn't handle the case of injection of GeneratorExit, but
  this won't be needed because throw_value == MP_OBJ_NULL

Note that it's already possible for mp_resume() to raise an exception
(including StopIteration) from the unprotected call to type->iternext(), so
that's why the VM already has code to handle the case of exceptions coming
out of mp_resume().

This commit reduces code size by a bit, and significantly reduces C stack
usage when using yield-from, from 88 bytes down to 40 for Thumb2, and 152
down to 72 bytes for x86-64 (better than half).  (Note that gcc doesn't
seem to tail-call optimise the call from mp_resume() to mp_obj_gen_resume()
so this saving in C stack usage helps all uses of yield-from.)

py/runtime.c

@@ -1248,15 +1248,8 @@ mp_vm_return_kind_t mp_resume(mp_obj_t self_in, mp_obj_t send_value, mp_obj_t th
     if (send_value == mp_const_none) {
         mp_load_method_maybe(self_in, MP_QSTR___next__, dest);
         if (dest[0] != MP_OBJ_NULL) {
-            nlr_buf_t nlr;
-            if (nlr_push(&nlr) == 0) {
-                *ret_val = mp_call_method_n_kw(0, 0, dest);
-                nlr_pop();
-                return MP_VM_RETURN_YIELD;
-            } else {
-                *ret_val = MP_OBJ_FROM_PTR(nlr.ret_val);
-                return MP_VM_RETURN_EXCEPTION;
-            }
+            *ret_val = mp_call_method_n_kw(0, 0, dest);
+            return MP_VM_RETURN_YIELD;
         }
     }
 
@@ -1265,10 +1258,6 @@ mp_vm_return_kind_t mp_resume(mp_obj_t self_in, mp_obj_t send_value, mp_obj_t th
     if (send_value != MP_OBJ_NULL) {
         mp_load_method(self_in, MP_QSTR_send, dest);
         dest[2] = send_value;
-        // TODO: This should have exception wrapping like __next__ case
-        // above. Not done right away to think how to optimize native
-        // generators better, see:
-        // https://github.com/micropython/micropython/issues/2628
         *ret_val = mp_call_method_n_kw(1, 0, dest);
         return MP_VM_RETURN_YIELD;
     }