Bignum: Remove/update obsolete comments

- We have moved to fixed window exponentiation and the algorithm used is
properly documented and referenced in core already, no need for
duplication.
- A comment on mbedtls_mpi_copy states that mbedtls_mpi_exp_mod relies
on it not to shrink X. This is not the case anymore, however we
should probably still state that some functions might rely on this
property as we don't know it for sure and it is safer to keep it that
way.

Signed-off-by: Janos Follath <janos.follath@arm.com>

library/bignum.c

@@ -288,8 +288,7 @@ static int mbedtls_mpi_resize_clear(mbedtls_mpi *X, size_t limbs)
  * This function is not constant-time. Leading zeros in Y may be removed.
  *
  * Ensure that X does not shrink. This is not guaranteed by the public API,
- * but some code in the bignum module relies on this property, for example
- * in mbedtls_mpi_exp_mod().
+ * but some code in the bignum module might still rely on this property.
  */
 int mbedtls_mpi_copy(mbedtls_mpi *X, const mbedtls_mpi *Y)
 {
@@ -1598,9 +1597,6 @@ int mbedtls_mpi_mod_int(mbedtls_mpi_uint *r, const mbedtls_mpi *A, mbedtls_mpi_s
     return 0;
 }
 
-/*
- * Sliding-window exponentiation: X = A^E mod N  (HAC 14.85)
- */
 int mbedtls_mpi_exp_mod(mbedtls_mpi *X, const mbedtls_mpi *A,
                         const mbedtls_mpi *E, const mbedtls_mpi *N,
                         mbedtls_mpi *prec_RR)