storage: Fix build with older compilers afeter gluster snapshot series

In commit e3226818 I accidentally added
twice a typedef for virStorageFileBackend when I moved it between files
across patch iterations. The double declaration breaks build on older
compilers in RHEL5 and FreeBSD.

Remove the spurious definition.

src/storage/storage_backend.h

@@ -207,9 +207,6 @@ typedef int
 (*virStorageFileBackendStat)(virStorageFilePtr file,
                              struct stat *st);
 
-typedef struct _virStorageFileBackend virStorageFileBackend;
-typedef virStorageFileBackend *virStorageFileBackendPtr;
-
 virStorageFileBackendPtr virStorageFileBackendForType(int type, int protocol);
 
 struct _virStorageFileBackend {