File the Fleet nav item under Assets, beside the PCs it describes

The endpoints plugin declared a section of its own, so the sidebar rendered a
one-item 'Fleet' group at the bottom, below Information, far from the PCs the
view is about. Its own docstring said it belonged just after PCs, and its
position (16, against PCs at 15) already said so too; only the section
disagreed.

Filed under assets, which puts it directly under PCs. The now-empty fleet
section is removed from NAV_SECTIONS - an unused section is an invitation to
file something else there.

frontend/src/views/AppLayout.vue

@@ -218,7 +218,6 @@ const NAV_SECTIONS = [
   { key: null, label: null },
   { key: 'assets', label: 'Assets' },
   { key: 'information', label: 'Information' },
-  { key: 'fleet', label: 'Fleet' },
 ]
 
 function buildNavItems(items) {