btrfs: remove redundant initialization of variable dirty in btrfs_update_time()

The variable dirty is initialized with a value that is never read, it
is being re-assigned later on. Remove the redundant initialization.
Cleans up clang scan build warning:

  fs/btrfs/inode.c:5965:7: warning: Value stored to 'dirty' during its
  initialization is never read [deadcode.DeadStores]
Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -6031,7 +6031,7 @@ static int btrfs_dirty_inode(struct btrfs_inode *inode)
 static int btrfs_update_time(struct inode *inode, int flags)
 {
 	struct btrfs_root *root = BTRFS_I(inode)->root;
-	bool dirty = flags & ~S_VERSION;
+	bool dirty;
 
 	if (btrfs_root_readonly(root))
 		return -EROFS;