No longer use SignalServiceAddress legacy identifier.

We had to do this in the past because we previously didn't allow
UUID-only contacts back in the day. This hasn't been the case for some
time. We should be preferring the UUID in all cases.

libsignal/service/src/main/java/org/whispersystems/signalservice/api/crypto/SignalServiceCipher.java

@@ -234,7 +234,7 @@ public class SignalServiceCipher {
     } else if (e164Address != null && store.containsSession(e164Address)) {
       return e164Address;
     } else {
-      return new SignalProtocolAddress(address.getLegacyIdentifier(), sourceDevice);
+      return new SignalProtocolAddress(address.getIdentifier(), sourceDevice);
     }
   }
 

libsignal/service/src/main/java/org/whispersystems/signalservice/api/push/SignalServiceAddress.java

@@ -69,16 +69,6 @@ public class SignalServiceAddress {
     }
   }
 
-  public String getLegacyIdentifier() {
-    if (e164.isPresent()) {
-      return e164.get();
-    } else if (uuid.isPresent()) {
-      return uuid.get().toString();
-    } else {
-      throw new AssertionError("Given the checks in the constructor, this should not be possible.");
-    }
-  }
-
   public Optional<String> getRelay() {
     return relay;
   }