Remove unreachable code

The returns can never be reached because the loop is guaranteed to return.

copy_from.go

@@ -334,6 +334,4 @@ func (c *Conn) CopyFromReader(r io.Reader, sql string) error {
 			return c.processContextFreeMsg(msg)
 		}
 	}
-
-	return nil
 }

copy_to.go

@@ -59,6 +59,4 @@ func (c *Conn) CopyToWriter(w io.Writer, sql string, args ...interface{}) error
 			return c.processContextFreeMsg(msg)
 		}
 	}
-
-	return nil
 }