Remove filter_term() shim function.

The filter_term() function was supporting the transition
from using tuples for search terms to using dictionaries,
but now all of the JS code should be dictionary-compatible.
(We had already abandoned the tuples safety net on staging,
and a couple days of use have given me confidence we can
pull the shim code.)

The one side effect this change has is that search terms will be
initialized to {} instead of [].  This distinction matters
when it comes to calling JSON.stringify on the search terms.

(imported from commit 1fbe11011d8953dbea28c0657cbf88384d343e00)

static/js/feature_flags.js

@@ -55,7 +55,6 @@ exports.propagate_topic_edits = true;
 exports.summarize_read_while_narrowed = false;
 exports.clicking_notification_causes_narrow = true;
 exports.use_socket = true;
-exports.remove_filter_tuples_safety_net = page_params.staging;
 
 // Ready for deprecation.
 exports.collapsible = false;