Removed stray assert, causing C90 build errors.

upb/reflection.c

@@ -67,7 +67,6 @@ static uint32_t *oneofcase(const upb_msg *msg,
 }
 
 static upb_msgval _upb_msg_getraw(const upb_msg *msg, const upb_fielddef *f) {
-  assert(f);
   const upb_msglayout_field *field = upb_fielddef_layout(f);
   const char *mem = PTR_AT(msg, field->offset, char);
   upb_msgval val = {0};