update logistration js to require json

This is Part 3 of clean-up for removing the shim from
LoginSessionView.post.  Not that the shim should be gone, the frontend
code can be simplified to always provide the shimless version of the
code.

ARCH-1253

lms/static/js/student_account/views/LoginView.js

@@ -189,14 +189,6 @@
             },
 
             saveError: function(error) {
-                if (error.responseJSON !== undefined) {
-                    this.saveErrorWithoutShim(error);
-                } else {
-                    this.saveErrorWithShim(error);
-                }
-            },
-
-            saveErrorWithoutShim: function(error) {
                 var errorCode;
                 var msg;
                 if (error.status === 0) {
@@ -233,41 +225,6 @@
                 this.toggleDisableButton(false);
             },
 
-            saveErrorWithShim: function(error) {
-                var msg = error.responseText;
-                if (error.status === 0) {
-                    msg = gettext('An error has occurred. Check your Internet connection and try again.');
-                } else if (error.status === 500) {
-                    msg = gettext('An error has occurred. Try refreshing the page, or check your Internet connection.'); // eslint-disable-line max-len
-                }
-                this.errors = [
-                    StringUtils.interpolate(
-                        '<li>{msg}</li>', {
-                            msg: msg
-                        }
-                    )
-                ];
-                this.clearPasswordResetSuccess();
-
-            /* If we've gotten a 403 error, it means that we've successfully
-             * authenticated with a third-party provider, but we haven't
-             * linked the account to an EdX account.  In this case,
-             * we need to prompt the user to enter a little more information
-             * to complete the registration process.
-             */
-                if (error.status === 403 &&
-                 error.responseText === 'third-party-auth' &&
-                 this.currentProvider) {
-                    if (!this.hideAuthWarnings) {
-                        this.clearFormErrors();
-                        this.renderThirdPartyAuthWarning();
-                    }
-                } else {
-                    this.renderErrors(this.defaultFormErrorsTitle, this.errors);
-                }
-                this.toggleDisableButton(false);
-            },
-
             renderThirdPartyAuthWarning: function() {
                 var message = _.sprintf(
                     gettext('You have successfully signed into %(currentProvider)s, but your %(currentProvider)s' +