filter: Remove redundant is:private operators.

If we have a pm-with, then is:private is redundant
and just forces us to write confusing/verbose code
in various places.

static/js/filter.js

@@ -152,7 +152,7 @@ function Filter(operators) {
     if (operators === undefined) {
         this._operators = [];
     } else {
-        this._operators = this._canonicalize_operators(operators);
+        this._operators = this.fix_operators(operators);
     }
 }
 
@@ -454,6 +454,26 @@ Filter.prototype = {
         return true;
     },
 
+    fix_operators: function (operators) {
+        operators = this._canonicalize_operators(operators);
+        operators = this._fix_redundant_is_private(operators);
+        return operators;
+    },
+
+    _fix_redundant_is_private: function (terms) {
+        const is_pm_with = (term) => {
+            return Filter.term_type(term) === 'pm-with';
+        };
+
+        if (!_.any(terms, is_pm_with)) {
+            return terms;
+        }
+
+        return _.reject(terms, (term) => {
+            return Filter.term_type(term) === 'is-private';
+        });
+    },
+
     _canonicalize_operators: function (operators_mixed_case) {
         return _.map(operators_mixed_case, function (tuple) {
             return Filter.canonicalize_term(tuple);