Remove EVP_MD_FLAG_PKEY_DIGEST

Update-Note: This flag is never set in BoringSSL, never checked by
external code, and hasn't even been defined in OpenSSL since 2015.

Change-Id: If3bf00f8ac02098b2ea1060f8cf601c2ec182265
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/78447
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Bob Beck <bbe@google.com>

include/openssl/digest.h

diff --git a/include/openssl/digest.h b/include/openssl/digest.h
index b36c588..9c12747 100644
--- a/include/openssl/digest.h
+++ b/include/openssl/digest.h
@@ -166,11 +166,6 @@
 // EVP_MD_block_size returns the native block-size of |md|, in bytes.
 OPENSSL_EXPORT size_t EVP_MD_block_size(const EVP_MD *md);
 
-// EVP_MD_FLAG_PKEY_DIGEST indicates that the digest function is used with a
-// specific public key in order to verify signatures. (For example,
-// EVP_dss1.)
-#define EVP_MD_FLAG_PKEY_DIGEST 1
-
 // EVP_MD_FLAG_DIGALGID_ABSENT indicates that the parameter type in an X.509
 // DigestAlgorithmIdentifier representing this digest function should be
 // undefined rather than NULL.