feat: remove obsolete model asset feature flags (#8566)

## Summary
Remove feature flags that are now enabled for all users and clean up
dead code paths.

## Changes

**Removed feature flags:**
- `huggingfaceModelImportEnabled` - always show generic URL input with
both Civitai/HuggingFace support
- `assetDeletionEnabled` - delete button always shown for mutable assets
- `asyncModelUploadEnabled` - always use async upload path (removed sync
upload and file size warnings)

**Kept feature flags:**
- `modelUploadButtonEnabled` - controls upload button visibility (local
vs cloud)
- `privateModelsEnabled` - controls upgrade modal vs upload dialog

**Deleted unused component:**
- `UploadModelUrlInputCivitai.vue` (replaced by generic input)

## Testing
- `pnpm typecheck` passes
- `pnpm lint` passes

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8566-feat-remove-obsolete-model-asset-feature-flags-2fc6d73d365081149ec6e78d995c8a44)
by [Unito](https://www.unito.io)


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

## Release Notes

* **Improvements**
  * Asset deletion option is now always available
  * Model upload experience simplified with unified import flow
  * Model uploads now consistently use asynchronous processing
  * HuggingFace model imports are now always supported

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Co-authored-by: Amp <amp@ampcode.com>

src/platform/assets/components/UploadModelDialogHeader.vue

@@ -1,11 +1,6 @@
 <template>
   <div class="flex items-center gap-2 p-4 font-bold">
-    <img
-      v-if="!flags.huggingfaceModelImportEnabled"
-      src="/assets/images/civitai.svg"
-      class="size-4"
-    />
-    <span>{{ $t(titleKey) }}</span>
+    <span>{{ $t('assetBrowser.uploadModelGeneric') }}</span>
     <span
       class="rounded-full bg-white px-1.5 py-0 text-xxs font-inter font-semibold uppercase text-black"
     >
@@ -13,17 +8,3 @@
     </span>
   </div>
 </template>
-
-<script setup lang="ts">
-import { computed } from 'vue'
-
-import { useFeatureFlags } from '@/composables/useFeatureFlags'
-
-const { flags } = useFeatureFlags()
-
-const titleKey = computed(() => {
-  return flags.huggingfaceModelImportEnabled
-    ? 'assetBrowser.uploadModelGeneric'
-    : 'assetBrowser.uploadModelFromCivitai'
-})
-</script>