fs: drop extra declaration of hexdump

hexdump was moved into a template in its own header years ago, but
the declaration of the implementation that used to be in fs.cc remains.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

include/crucible/fs.h

@@ -243,8 +243,6 @@ namespace crucible {
 		unsigned long available() const;
 	};
 
-	template<class V> ostream &hexdump(ostream &os, const V &v);
-
 	struct BtrfsIoctlFsInfoArgs : public btrfs_ioctl_fs_info_args_v3 {
 		BtrfsIoctlFsInfoArgs();
 		void do_ioctl(int fd);