fix assign grammar of application agent

[jidalong]

What type of PR is this?
/kind bug

What this PR does / why we need it:
Since variable "defaultAgent" was assigned by ":=" in line 273, so we should delete the declared line 268.
Does this PR introduce a user-facing change?:
NONE

[gy95]

/LGTM

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In response to this:

/LGTM

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[fisherxu]

It seems that we can delete the global variable, and keep the local variable here.

[jidalong]

Okay~ I have delete the global variable instead.

[zc2638]

var once sync.Once

According to the NewApplicationAgent method, we probably shouldn't use once to execute.

[jidalong]

var once sync.Once

According to the NewApplicationAgent method, we probably shouldn't use once to execute.

@zc2638 Should we just change the part of NewApplicationAgent method

once.Do(func() {
  go wait.Until(func() {
  	defaultAgent.GC()
  }, time.Minute*5, beehiveContext.Done())
})

to the flowing code, and delete the declare "var once sync.Once".

go wait.Until(func() {
    defaultAgent.GC()
}, time.Minute*5, beehiveContext.Done())

[zc2638]

@jidalong
I think so.

[jidalong]

@jidalong I think so.

Okey, I think it would be better as well~and has change it.

[zc2638]

/lgtm
/assign @fisherxu

[fisherxu]

/approve

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