fix: remove invalidateAvatarCache call that broke request deduplication (#2317)

When multiple avatar components mount with alternating sizes (e.g. 48
and 20), invalidateAvatarCache clears pending requests for ALL sizes of
the same user, causing each call to create a new HTTP request instead of
reusing the pending one.

I noticed this issue when I open a task with 20 comments, the avatar
requests make the service OOM.

<img width="733" height="551" alt="image"
src="https://github.com/user-attachments/assets/db45db31-294c-4363-ad27-38d454b5a6a2"
/>

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

frontend/src/models/user.ts

@@ -24,9 +24,7 @@ export async function fetchAvatarBlobUrl(user: IUser, size = 50) {
 	if (pendingRequests.has(key)) {
 		return await pendingRequests.get(key) as string
 	}
-	
-	invalidateAvatarCache(user)
-	
+
 	// Create a new request
 	const requestPromise = avatarService.getBlobUrl(`/avatar/${user.username}?size=${size}`)
 		.then(url => {