Remove unmount_tree in VolumeManager.

This is not needed anymore since everything else is updated
to use android::vold::UnmountTree.

Bug: 111890351
Test: manual
Change-Id: Idb574469763fa3438fbfc40d6da786d87ac63c38
diff --git a/VolumeManager.cpp b/VolumeManager.cpp
index db47e4d..cbe3187 100644
--- a/VolumeManager.cpp
+++ b/VolumeManager.cpp
@@ -410,10 +410,6 @@
     return 0;
 }
 
-static int unmount_tree(const std::string& prefix) {
-    return android::vold::UnmountTree(prefix);
-}
-
 int VolumeManager::remountUid(uid_t uid, const std::string& mode) {
     LOG(DEBUG) << "Remounting " << uid << " as mode " << mode;
 
@@ -483,7 +479,7 @@
                 _exit(1);
             }
 
-            unmount_tree("/storage/");
+            android::vold::UnmountTree("/storage/");
 
             std::string storageSource;
             if (mode == "default") {