md/r5cache: remove redundant pointer bio

Pointer bio is being assigned but is never used hence it is redundant
and can be removed.

Cleans up clang warning:
warning: variable 'bio' set but not used [-Wunused-but-set-variable]

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Shaohua Li <shli@fb.com>