remove the "save all answers" button

The "save answer" button is shown after each part, even when it's the
only one in the question.

This also removes the "save all answers" button in the questoin nav bar
at the bottom.

fixes #1118

themes/default/files/scripts/part-display.js

@@ -347,9 +347,6 @@ Numbas.queueScript('part-display',['display-util', 'display-base','util','jme'],
          * @memberof Numbas.display.PartDisplay
          */
         this.showSubmitPart = Knockout.computed(function() {
-            if(p.question && p.question.partsMode!='explore' && p.question.display.parts().length<=1 && !p.isStep) {
-                return false;
-            }
             return this.doesMarking() && !this.disabled();
         },this);
         /** Text to describe the state of the steps penalty.

themes/default/templates/question-nav.xslt

@@ -2,7 +2,6 @@
 <!-- bottom nav bar - prv/nxt, plus submit/advice/reveal buttons -->
 <nav class="question-nav question-bottom-nav navbar navbar-default" data-bind="jmescope: question.scope,attr: {{'aria-label': R('question.nav.label')}}">
     <div class="nav navbar-nav">
-        <button class="btn btn-primary navbar-btn submitBtn" data-bind="visible: $root.exam().mode()=='normal' &amp;&amp; question.partsMode=='all' &amp;&amp; !revealed() &amp;&amp; question.parts.length, click: Numbas.controls.submitQuestion, text: submitMessage"><localise>control.submit all parts</localise></button>
         <p class="marks navbar-text" data-bind="visible: !showScoreBreakdown()">
             <span class="score" data-bind="html: scoreFeedback.message, pulse: scoreFeedback.update"></span>
             <span class="feedback-icon" data-bind="css: scoreFeedback.iconClass, attr: scoreFeedback.iconAttr, pulse: scoreFeedback.update" aria-hidden="true"></span>