Remove glad and replace it with our own GL header

This completely removes the dependency on glad by generating the GL
headers from gl.xml directly.

This requires adding khrplatform.h so all Khronos dependencies are
gathered in third_party/khronos.

Also removes a stray CMakeLists.txt that was still hanging out.

BUG=dawn:165

Change-Id: Ia64bc51bc8b18c6b48613918e2f309f7405ecb3b
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/8163
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>

generator/templates/opengl/OpenGLFunctionsBase.h

@@ -13,8 +13,7 @@
 //* limitations under the License.
 
 #include "dawn_native/Error.h"
-
-#include <glad/glad.h>
+#include "dawn_native/opengl/opengl_platform.h"
 
 namespace dawn_native { namespace opengl {
     using GetProcAddress = void* (*) (const char*);
@@ -24,7 +23,7 @@ namespace dawn_native { namespace opengl {
         {% for block in header_blocks %}
             // {{block.description}}
             {% for proc in block.procs %}
-                {{proc.PFNPROCNAME()}} {{proc.ProcName()}} = nullptr;
+                {{proc.PFNGLPROCNAME()}} {{proc.ProcName()}} = nullptr;
             {% endfor %}
 
         {% endfor%}