refactor: start on removing FF for subscription tiers (#7596)

## Summary

Refactor: remove FF for subscription tier, remove legacy code for non
subscription tier logic.
 
## Review Focus

Preexisting cloud functionality impact.

<!-- If this PR fixes an issue, uncomment and update the line below -->
<!-- Fixes #ISSUE_NUMBER -->

## Screenshots (if applicable)

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┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-7596-refactor-start-on-removing-FF-for-subscription-tiers-2cc6d73d3650816bac3aef893e4f37cd)
by [Unito](https://www.unito.io)

src/components/topbar/CurrentUserPopover.test.ts

@@ -138,15 +138,6 @@ vi.mock('@/composables/useExternalLink', () => ({
   }))
 }))
 
-// Mock useFeatureFlags
-vi.mock('@/composables/useFeatureFlags', () => ({
-  useFeatureFlags: vi.fn(() => ({
-    flags: {
-      subscriptionTiersEnabled: true
-    }
-  }))
-}))
-
 // Mock useTelemetry
 vi.mock('@/platform/telemetry', () => ({
   useTelemetry: vi.fn(() => ({

src/components/topbar/CurrentUserPopover.vue

@@ -42,7 +42,6 @@
         formattedBalance
       }}</span>
       <i
-        v-if="flags.subscriptionTiersEnabled"
         v-tooltip="{ value: $t('credits.unified.tooltip'), showDelay: 300 }"
         class="icon-[lucide--circle-help] cursor-help text-base text-muted-foreground mr-auto"
       />
@@ -147,7 +146,6 @@ import UserAvatar from '@/components/common/UserAvatar.vue'
 import { useCurrentUser } from '@/composables/auth/useCurrentUser'
 import { useFirebaseAuthActions } from '@/composables/auth/useFirebaseAuthActions'
 import { useExternalLink } from '@/composables/useExternalLink'
-import { useFeatureFlags } from '@/composables/useFeatureFlags'
 import SubscribeButton from '@/platform/cloud/subscription/components/SubscribeButton.vue'
 import { useSubscription } from '@/platform/cloud/subscription/composables/useSubscription'
 import { useSubscriptionDialog } from '@/platform/cloud/subscription/composables/useSubscriptionDialog'
@@ -174,7 +172,6 @@ const {
   fetchStatus
 } = useSubscription()
 const subscriptionDialog = useSubscriptionDialog()
-const { flags } = useFeatureFlags()
 const { locale } = useI18n()
 
 const formattedBalance = computed(() => {