Cleanup: Clang-Tidy readability-redundant-preprocessor fixes

Remove redundantly nested `#if` and `#ifdef` statements.

One nested `#if 0` block was left untouched, as it's in particle code
that's no longer maintained. Furthermore, that block also has some
explanation as to the differences between the enabled & disabled parts.

One nested `#if 0` construct was completely removed, leaving only the
actually used bit of code. There was no explanation as to the usefulness
of the disabled code, and it hasn't been touched in years.

No functional changes.

.clang-tidy

@@ -13,7 +13,6 @@ Checks:  >
   -readability-misleading-indentation,
 
   -readability-inconsistent-declaration-parameter-name,
-  -readability-redundant-preprocessor,
   -readability-redundant-member-init,
   -readability-const-return-type,
   -readability-static-accessed-through-instance,