Get rid of useless VIR_STORAGE_FILE_FEATURE_NONE

It's not used anywhere except for the switch in
virStorageBackendCreateQemuImgOpts, where leaving it in causes
a dead code coverity warning and omitting it breaks compilation
because of unhandled enum value.

Introduced by 6298f74.

src/storage/storage_backend.c

@@ -666,7 +666,7 @@ virStorageBackendCreateQemuImgOpts(char **opts,
                         goto error;
                     }
                     break;
-                case VIR_STORAGE_FILE_FEATURE_NONE:
+
                 case VIR_STORAGE_FILE_FEATURE_LAST:
                     ;
                 }

src/util/virstoragefile.h

@@ -53,8 +53,7 @@ enum virStorageFileFormat {
 VIR_ENUM_DECL(virStorageFileFormat);
 
 enum virStorageFileFeature {
-    VIR_STORAGE_FILE_FEATURE_NONE = -1,
+    VIR_STORAGE_FILE_FEATURE_LAZY_REFCOUNTS = 0,
-    VIR_STORAGE_FILE_FEATURE_LAZY_REFCOUNTS,
 
     VIR_STORAGE_FILE_FEATURE_LAST
 };