Fix warning about unused |EVP_PKEY_METHOD.ctrl_str|.

Some compilers in some configurations warn about this structure member
not being assigned a value. Since it is never used anywhere, just
remove it.

Change-Id: I46064234961bf449fe5fcb88594ddb3ff390e7d7
Reviewed-on: https://boringssl-review.googlesource.com/5621
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/crypto/evp/internal.h b/crypto/evp/internal.h
index 4e1e9c9..60881e3 100644
--- a/crypto/evp/internal.h
+++ b/crypto/evp/internal.h
@@ -261,7 +261,6 @@
   int (*derive)(EVP_PKEY_CTX *ctx, uint8_t *key, size_t *keylen);
 
   int (*ctrl)(EVP_PKEY_CTX *ctx, int type, int p1, void *p2);
-  int (*ctrl_str)(EVP_PKEY_CTX *ctx, const char *type, const char *value);
 } /* EVP_PKEY_METHOD */;