fix(plg): remove summit-plg handler flag check in favour of entitlement

[Kanishkavijay39]

The isHandlerEnabledForSite('summit-plg') feature flag is no longer needed as the PLG entitlement tier is already the single source of truth for PLG access. Removing the flag simplifies the check and eliminates a stale dependency.

Resolves: SITES-44095

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