It started out as busywork, but it does many separate things. If I could
start over, I'd take them apart into multiple commits:

1. Remove G_ENABLE_DEBUG around GDK_DEBUG_*() calls
   This is not needed at all, the calls themselves take care of it.

2. Remove G_ENABLE_DEBUG around profiling code
   This now enables profiling support in release builds.

3. Stop poking _gdk_debug_flags and use GDK_DEBUG_CHECK()
   This was old code that was never updated.

4. Make !G_ENABLE_DEBUG turn off GDK_DEBUG_CHECK()
   The code used to
     #define GDK_DEBUG_CHECK(...) false
     #define GDK_DEBUG(...)
   which would compile away all the code inside those macros. This
   means a lot of variable definitions and debug utility functions
   would suddenly no longer be used and cause compiler errors.