[core] Fix bugprone-forward-declaration-namespace errors in header files

As reported by clang-tidy-8.

include/mbgl/gl/renderer_backend.hpp

@@ -8,7 +8,6 @@
 namespace mbgl {
 namespace gl {
 
-class Context;
 using ProcAddress = void (*)();
 using FramebufferID = uint32_t;
 

src/mbgl/renderer/buckets/debug_bucket.hpp

@@ -14,10 +14,6 @@ namespace mbgl {
 
 class OverscaledTileID;
 
-namespace gl {
-class UploadPass;
-} // namespace gl
-
 class DebugBucket : private util::noncopyable {
 public:
     DebugBucket(const OverscaledTileID& id,