Remove X509at_get0_data_by_OBJ.

This function's behavior differs from all the other lastpos functions.
It does not appear to be used anywhere, so remove it. (lastpos = -1
returns the first match, lastpos = -2 additionally fails if there are
duplicates, lastpos = -3 additionally fails if the attribute is
multiply-valued.)

Update-Note: X509at_get0_data_by_OBJ is removed. We found no callers of
this function.

Change-Id: I8547bac6626623e43827e2490f04850eb148e317
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/48367
Reviewed-by: Adam Langley <agl@google.com>

crypto/x509/x509_att.c

diff --git a/crypto/x509/x509_att.c b/crypto/x509/x509_att.c
index e7e6227..d3e78c3 100644
--- a/crypto/x509/x509_att.c
+++ b/crypto/x509/x509_att.c
@@ -204,22 +204,6 @@
     return ret;
 }
 
-void *X509at_get0_data_by_OBJ(STACK_OF(X509_ATTRIBUTE) *x,
-                              ASN1_OBJECT *obj, int lastpos, int type)
-{
-    int i;
-    X509_ATTRIBUTE *at;
-    i = X509at_get_attr_by_OBJ(x, obj, lastpos);
-    if (i == -1)
-        return NULL;
-    if ((lastpos <= -2) && (X509at_get_attr_by_OBJ(x, obj, i) != -1))
-        return NULL;
-    at = X509at_get_attr(x, i);
-    if (lastpos <= -3 && (X509_ATTRIBUTE_count(at) != 1))
-        return NULL;
-    return X509_ATTRIBUTE_get0_data(at, 0, type, NULL);
-}
-
 X509_ATTRIBUTE *X509_ATTRIBUTE_create_by_NID(X509_ATTRIBUTE **attr, int nid,
                                              int attrtype, const void *data,
                                              int len)

include/openssl/x509.h

diff --git a/include/openssl/x509.h b/include/openssl/x509.h
index 564d102..21cb696 100644
--- a/include/openssl/x509.h
+++ b/include/openssl/x509.h
@@ -1796,12 +1796,6 @@
     STACK_OF(X509_ATTRIBUTE) **x, const char *attrname, int type,
     const unsigned char *bytes, int len);
 
-// TODO(davidben): Document or remove this function. The behavior of |lastpos|
-// is complex.
-OPENSSL_EXPORT void *X509at_get0_data_by_OBJ(STACK_OF(X509_ATTRIBUTE) *x,
-                                             ASN1_OBJECT *obj, int lastpos,
-                                             int type);
-
 // X509_ATTRIBUTE_create_by_NID returns a newly-allocated |X509_ATTRIBUTE| of
 // type |nid|, or NULL on error. The value is determined as in
 // |X509_ATTRIBUTE_set1_data|.