remove get affected rows check

In MySQL result, affected rows means the number of rows that get
updated, but not the number of matched rows. So this can't be use as an
indicator that if the rows actually exist.

Author: wwwjfy

src/gino/crud.py

@@ -824,8 +824,6 @@ async def _execute_and_fetch(conn, query):
                 )
                 row = await conn.first(query)
             elif context.isupdate:
-                if context.get_affected_rows() == 0:
-                    raise NoSuchRowError()
                 table = context.compiled.statement.table
                 if len(table.primary_key) > 0:
                     lookup_conds = [

src/gino/dialects/base.py

@@ -414,6 +414,8 @@ def get_lastrowid(self):
         raise NotImplementedError
 
     def get_affected_rows(self):
+        # Note: in MySQL result, affected rows means the number of rows that get
+        # updated, but not the number of matched rows.
         raise NotImplementedError