[SPARK-14077][ML][FOLLOW-UP] Revert change for NB Model's Load to maintain...

[SPARK-14077][ML][FOLLOW-UP] Revert change for NB Model's Load to maintain compatibility with the model stored before 2.0

## What changes were proposed in this pull request?
Revert change for NB Model's Load to maintain compatibility with the model stored before 2.0

## How was this patch tested?
local build

Author: Zheng RuiFeng <ruifengz@foxmail.com>

Closes #15313 from zhengruifeng/revert_save_load.