Revert "Replace DesignKit with package from ElementX."

This reverts the following commits:
ef5365ab240a1449e0490d2eb011dd69f594e27b
545b641e53a845b722f571b48ab408000048714b
702b7a696dd1a8319d6af907d4766a035cf30234
2398c1534dd1ca5b6329c888c50f55fb1b0ec23f

DesignKit/DesignKit.h

@@ -0,0 +1,27 @@
+// 
+// Copyright 2021 New Vector Ltd
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+#import <Foundation/Foundation.h>
+
+//! Project version number for DesignKit.
+FOUNDATION_EXPORT double DesignKitVersionNumber;
+
+//! Project version string for DesignKit.
+FOUNDATION_EXPORT const unsigned char DesignKitVersionString[];
+
+// In this header, you should import all the public headers of your framework using statements like #import <DesignKit/PublicHeader.h>
+
+