This partially reverts commit a8ea8bdd.

Most of it is no longer needed since sm2 has been removed.  However,
the following functions have been kept as they have developed other
uses:

mpi_copy

mpi_mod

mpi_test_bit
mpi_set_bit
mpi_rshift

mpi_add
mpi_sub
mpi_addm
mpi_subm

mpi_mul
mpi_mulm

mpi_tdiv_r
mpi_fdiv_r
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>