SCons: Cleanup DEBUG, _DEBUG and NDEBUG defines

- `_DEBUG` is MSVC specific so it didn't make much sense to define for
  Android and iOS builds.
- iOS was the only platform to define `DEBUG`. We don't use it anywhere
  outside thirdparty code, which we usually don't intend to debug, so it
  seems better to be consistent with other platforms.
- Consistently define `NDEBUG` to disable assert behavior in both `release`
  and `release_debug` targets. This used to be set for `release` for all
  platforms, and `release_debug` for Android and iOS only.
- Due to the above, I removed the only use we made of `assert()` in Godot
  code, which was only implemented for Unix anyway, should have been
  `DEV_ENABLED`, and is in PoolAllocator which we don't actually use.
- The denoise and recast modules keep defining `NDEBUG` even for the `debug`
  target as we don't want OIDN and Embree asserting all over the place.

core/os/pool_allocator.cpp

@@ -35,8 +35,6 @@
 #include "core/os/os.h"
 #include "core/string/print_string.h"
 
-#include <assert.h>
-
 #define COMPACT_CHUNK(m_entry, m_to_pos)                      \
 	do {                                                      \
 		void *_dst = &((unsigned char *)pool)[m_to_pos];      \
@@ -169,11 +167,6 @@ bool PoolAllocator::find_entry_index(EntryIndicesPos *p_map_pos, const Entry *p_
 
 PoolAllocator::ID PoolAllocator::alloc(int p_size) {
 	ERR_FAIL_COND_V(p_size < 1, POOL_ALLOCATOR_INVALID_ID);
-#ifdef DEBUG_ENABLED
-	if (p_size > free_mem) {
-		OS::get_singleton()->debug_break();
-	}
-#endif
 	ERR_FAIL_COND_V(p_size > free_mem, POOL_ALLOCATOR_INVALID_ID);
 
 	mt_lock();
@@ -482,7 +475,6 @@ void *PoolAllocator::get(ID p_mem) {
 		ERR_FAIL_COND_V(!e, nullptr);
 	}
 	if (e->lock == 0) {
-		//assert(0);
 		mt_unlock();
 		ERR_PRINT("e->lock == 0");
 		return nullptr;