Drop support for engines-provided signature verification.

We do not need to support engine-provided verification methods.

Change-Id: Iaad8369d403082b728c831167cc386fdcabfb067
Reviewed-on: https://boringssl-review.googlesource.com/7311
Reviewed-by: David Benjamin <davidben@google.com>
diff --git a/include/openssl/rsa.h b/include/openssl/rsa.h
index 19325aa..ea42525 100644
--- a/include/openssl/rsa.h
+++ b/include/openssl/rsa.h
@@ -509,6 +509,7 @@
   int (*sign)(int type, const uint8_t *m, unsigned int m_length,
               uint8_t *sigret, unsigned int *siglen, const RSA *rsa);
 
+  /* Ignored. Set this to NULL. */
   int (*verify)(int dtype, const uint8_t *m, unsigned int m_length,
                 const uint8_t *sigbuf, unsigned int siglen, const RSA *rsa);
 
@@ -521,6 +522,7 @@
 
   int (*decrypt)(RSA *rsa, size_t *out_len, uint8_t *out, size_t max_out,
                  const uint8_t *in, size_t in_len, int padding);
+  /* Ignored. Set this to NULL. */
   int (*verify_raw)(RSA *rsa, size_t *out_len, uint8_t *out, size_t max_out,
                     const uint8_t *in, size_t in_len, int padding);