Cleanup datatable code in sysadmin user tab

There is no significant impact to this change;
it's just some general cleanup that I made while cleaning up this file..

We don't need to store these as instance variables,
since we never use them as such.
And since we've already factored out the remaining code from this
initialization method, we can drop it entirely.

lms/djangoapps/dashboard/sysadmin.py

@@ -173,24 +173,30 @@ class Users(SysadminDashboardView):
         user.delete()
         return _(u'Deleted user {username}').format(username=uname)
 
-    def make_common_context(self):
-        """Returns the datatable used for this view"""
-
-        self.datatable = {}
-
-        self.datatable = dict(header=[_('Statistic'), _('Value')],
-                              title=_('Site statistics'))
-        self.datatable['data'] = [[_('Total number of users'),
-                                   User.objects.all().count()]]
+    def make_datatable(self):
+        """
+        Build the datatable for this view
+        """
+        datatable = {
+            'header': [
+                _('Statistic'),
+                _('Value'),
+            ],
+            'title': _('Site statistics'),
+            'data': [
+                [
+                    _('Total number of users'),
+                    User.objects.all().count(),
+                ],
+            ],
+        }
+        return datatable
 
     def get(self, request):
-
         if not request.user.is_staff:
             raise Http404
-        self.make_common_context()
-
         context = {
-            'datatable': self.datatable,
+            'datatable': self.make_datatable(),
             'msg': self.msg,
             'djangopid': os.getpid(),
             'modeflag': {'users': 'active-section'},
@@ -203,9 +209,6 @@ class Users(SysadminDashboardView):
 
         if not request.user.is_staff:
             raise Http404
-
-        self.make_common_context()
-
         action = request.POST.get('action', '')
         track.views.server_track(request, action, {}, page='user_sysdashboard')
 
@@ -226,9 +229,8 @@ class Users(SysadminDashboardView):
             uname = request.POST.get('student_uname', '').strip()
             self.msg = HTML(u'<h4>{0}</h4><p>{1}</p><hr />{2}').format(
                 _('Delete User Results'), self.delete_user(uname), self.msg)
-
         context = {
-            'datatable': self.datatable,
+            'datatable': self.make_datatable(),
             'msg': self.msg,
             'djangopid': os.getpid(),
             'modeflag': {'users': 'active-section'},