do_mpage_readpage(): remove useless clear_buffer_mapped() call

It is known that buffer_mapped() is false in this code path.

Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>