test: remove outdated empty files assertion from clean_empty_library_items test

The clean_empty_library_items function intentionally skips empty file cleanup
to avoid removing app sentinel files (as noted in lib/clean/user.sh:77).
Updated the test to only verify empty folder cleanup, matching current behavior.

tests/clean_user_core.bats

@@ -117,7 +117,6 @@ EOF
 
     [ "$status" -eq 0 ]
     [[ "$output" == *"Empty Library folders"* ]]
-    [[ "$output" == *"Empty Library files"* ]]
 }
 
 @test "clean_browsers calls expected cache paths" {