Remove the API setBridgePortLearningFDB() and use setBridgePortLearnMode() only

[DennisChiuEC]

What I did
1.Remove the API setBridgePortLearningFDB()
2.Use setBridgePortLearnMode() as instead
3.Set PHY PORT's learning mode to forward during swss warm restart
which is derived from the setBridgePortLearningFDB() API called by swss warm restart.

Why I did it
Two API setBridgePortLearningFDB() and setBridgePortLearnMode() do the same thing which configured port learning mode.
The code can be refined to have only one API.

How I verified it

Details if related

[prsunny]

@DennisChiuEC , understand the motivation to use one api. What is the other change during warmboot? Is it fixing any issue? @dgsudharsan , @vaibhavhd for viz

[DennisChiuEC]

The other change is aimed to make the L2 portchannel can forward packet during warmboot.
Maybe I should separate the change into another PR?
And this PR is focus on the change to use one api to set port learn mode.

[prsunny]

The other change is aimed to make the L2 portchannel can forward packet during warmboot. Maybe I should separate the change into another PR? And this PR is focus on the change to use one api to set port learn mode.

that would be great

[DennisChiuEC]

The other change is aimed to make the L2 portchannel can forward packet during warmboot. Maybe I should separate the change into another PR? And this PR is focus on the change to use one api to set port learn mode.

that would be great

Done, has removed the change which is not related to refine the API.