Remove unused static variable.

Test: Builds with -Wunused-variable
Change-Id: I65c6dbad7fa075e912dbd63f871faf4c0a0c95fc
diff --git a/VolumeManager.cpp b/VolumeManager.cpp
index 2171717..7c2b2f2 100644
--- a/VolumeManager.cpp
+++ b/VolumeManager.cpp
@@ -101,8 +101,6 @@
 
 static const char* kPropVirtualDisk = "persist.sys.virtual_disk";
 
-static const std::string kEmptyString("");
-
 /* 512MiB is large enough for testing purposes */
 static const unsigned int kSizeVirtualDisk = 536870912;
 
@@ -1311,4 +1309,4 @@
 
     *storageSize *= 512;
     return OK;
-}
\ No newline at end of file
+}