Stop using -Wno-unused-variable

This is a useful warning, so remove two unused variables and stop
disabling this warning.

Change-Id: I3efe0b300ea139ea11d645dec6fb2613acc0e51b

Android.bp

@@ -11,7 +11,6 @@ cc_defaults {
         "-Wextra",
         "-Wno-missing-field-initializers",
         "-Wno-unused-parameter",
-        "-Wno-unused-variable",
     ],
 
     clang: true,

VolumeManager.cpp

@@ -93,7 +93,6 @@ using android::vold::UnmountTree;
 using android::vold::VoldNativeService;
 using android::vold::VolumeBase;
 
-static const char* kPathUserMount = "/mnt/user";
 static const char* kPathVirtualDisk = "/data/misc/vold/virtual_disk";
 
 static const char* kPropVirtualDisk = "persist.sys.virtual_disk";

fs/Ext4.cpp

@@ -66,8 +66,6 @@ status_t Check(const std::string& source, const std::string& target) {
 
     const char* c_source = source.c_str();
     const char* c_target = target.c_str();
-
-    int status;
     int ret;
     long tmpmnt_flags = MS_NOATIME | MS_NOEXEC | MS_NOSUID;
     char* tmpmnt_opts = (char*)"nomblk_io_submit,errors=remount-ro";