lib/mpi: purge mpi_set_buffer()

mpi_set_buffer() has no in-tree users and similar functionality is provided
by mpi_read_raw_data().

Remove mpi_set_buffer().

Signed-off-by: Nicolai Stange <nicstange@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>