remove obsolete BN_CTX info

doc/crypto/bn_internal.pod

@@ -92,18 +92,8 @@ Various routines in this library require the use of temporary
 B<BIGNUM> variables during their execution.  Since dynamic memory
 allocation to create B<BIGNUM>s is rather expensive when used in
 conjunction with repeated subroutine calls, the B<BN_CTX> structure is
-used.  This structure contains B<BN_CTX_NUM> B<BIGNUM>s.
-B<BN_CTX_NUM> is the maximum number of temporary B<BIGNUM>s any
-publicly exported function will use.
-
- #define BN_CTX_NUM  12
- typedef struct bignum_ctx
-        {
-        int tos;                    /* top of stack */
-        BIGNUM *bn[BN_CTX_NUM];     /* The variables */
-        } BN_CTX;
-
-B<tos> is the index of the first unused B<BIGNUM> in the B<bn> array.
+used.  This structure contains B<BN_CTX_NUM> B<BIGNUM>s, see
+L<BN_CTX_start(3)|BN_CTX_start(3)>.
 
 =head2 Low-level arithmetic operations