Eliminate redundant and obsolete cruft from dnf_keyring_add_public_key()

These APIs have been obsolete for years and now removed in rpm 4.19.
Luckily the usage here is entirely redundant, rpmKeyringAddKey() checks
for duplicates and that case is even handled in this code.

libdnf/dnf-keyring.cpp

@@ -63,7 +63,6 @@ dnf_keyring_add_public_key(rpmKeyring keyring,
     int rc;
     gsize len;
     pgpArmor armor;
-    pgpDig dig = NULL;
     rpmPubkey pubkey = NULL;
     rpmPubkey *subkeys = NULL;
     int nsubkeys = 0;
@@ -116,16 +115,7 @@ dnf_keyring_add_public_key(rpmKeyring keyring,
         goto out;
     }
 
-    /* does the key exist in the keyring */
-    dig = rpmPubkeyDig(pubkey);
-    rc = rpmKeyringLookup(keyring, dig);
-    if (rc == RPMRC_OK) {
-        ret = TRUE;
-        g_debug("%s is already present", filename);
-        goto out;
-    }
-
-    /* add to rpmdb automatically, without a prompt */
+    /* add to in-memory keyring */
     rc = rpmKeyringAddKey(keyring, pubkey);
     if (rc == 1) {
         ret = TRUE;
@@ -169,8 +159,6 @@ dnf_keyring_add_public_key(rpmKeyring keyring,
         }
         free(subkeys);
     }
-    if (dig != NULL)
-        pgpFreeDig(dig);
     return ret;
 } CATCH_TO_GERROR(FALSE)