Fix memory-hog query in AssetCountForSidebar middleware

[jerm]

https://github.com/snipe/snipe-it/pull/14702/files introduced a bug
where instead of doing a quick select count(*) of assets, it did a select * of
assets, moving the count from the database to the PHP process.

This caused OOM issues in memory-constrained environments with lots of
assets, and also presented a speed issue even when memory limited were
increased.

Additionally, given this populates the sidebar, this was likely an issue
on every page load that included the sidebar.

The fix is simply removing the all()->, ending up with Asset::count(),
which yields the desired select count(*) DB query.

[what-the-diff]

PR Summary

• Optimized Asset Count Calculation
  The modification in the AssetCountForSidebar.php file changes our method of counting the total number of assets. Previously, we were getting all assets then counting them, which was less efficient. The new method simply counts the assets directly, enhancing overall performance of the sidebar where this total count is displayed.