Remove FlavorFungibilityImplicitPreferenceDefault feature gate.

[mbobrovskyi]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

Remove FlavorFungibilityImplicitPreferenceDefault feature gate.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?

Removed FlavorFungibilityImplicitPreferenceDefault feature gate.

ACTION REQUIRED: Configure flavor selection preference using the ClusterQueue field `spec.flavorFungibility.preference` instead.

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[mimowo]

Actually when FlavorFungibilityImplicitPreferenceDefault was enabled and
WhenCanBorrow: TryNextFlavor
WhenCanPreempt: MayStopSearch

then we we would choose preemptionOverBorrowing. Now we will use borrowingOverPreemption.

I think it makes sense to use preemptionOverBorrowing when this configuration is used. Wdyt @tenzen-y @pajakd ?

cc @amy