[data-poll-handler] always process data poll when acked with "frame pending" (#4131)

This commit removes the extra check remaining from before the change
from #3785 and the related follow-ups. Before having the "Acked with
FP" feature, we relied on number of indirect msg in queue (for the
child) or state of src-match-table to determine/guess if the data poll
was acked with fp or not. That is now redundant and can be incorrect
(e.g., if a message was removed/evicted after data poll was acked with
fp set).

If the data poll is acked with "frame pending" flag set, the child
would stay in rx expecting a frame from parent. If we have no frame
for the child, parent would send an empty frame to let the child know
that it can go to sleep (and avoid the child having a data timeout and
retx the data poll).

This change is required for new platform abstractions like Thread over
BLE (ToBLE) where we may not have the ack or frame-pending notion at
the link layer and rely on next layer to follow a "data poll" with
empty frame (when there is no pending frame).

src/core/mac/data_poll_handler.cpp

@@ -154,8 +154,6 @@ void DataPollHandler::HandleDataPoll(Mac::RxFrame &aFrame)
         ExitNow();
     }
 
-    VerifyOrExit(!Get<SourceMatchController>().IsEnabled() || (indirectMsgCount > 0));
-
     if (mIndirectTxChild == NULL)
     {
         mIndirectTxChild = child;