lib/model: Use a single lock (phase two: cleanup) (#9276)

Cleanup after #9275.

This renames `fmut` -> `mut`, removes the deadlock detector and
associated plumbing, renames some things from `...PRLocked` to
`...RLocked` and similar, and updates comments.

Apart from the removal of the deadlock detection machinery, no
functional code changes... i.e. almost 100% diff noise, have fun
reviewing.

lib/model/folder_recvonly_test.go

@@ -535,8 +535,8 @@ func setupROFolder(t *testing.T) (*testModel, *receiveOnlyFolder, context.Cancel
 	<-m.started
 	must(t, m.ScanFolder("ro"))
 
-	m.fmut.RLock()
-	defer m.fmut.RUnlock()
+	m.mut.RLock()
+	defer m.mut.RUnlock()
 	r, _ := m.folderRunners.Get("ro")
 	f := r.(*receiveOnlyFolder)