Delete src/thread/n3ds/SDL_syscond.c

This PR removes the incorrect implementation of `SDL_cond` currently included with the 3DS port.

Pseudocode of the incorrect implementation of `SDL_CondWait` this PR removes:

* Receive an `SDL_cond` backed by a `libctru` `CondVar` and an `SDL_mutex` backed by a `libctru` `RecursiveLock`.
* Want to call `libctru` function `CondVar_Wait` which expects a `CondVar` and a `LightLock` (non-recursive lock)
* Do so by calling this function with the internal (inadequately protected) `LightLock` member of the `RecursiveLock` (`&mutex->lock.lock` on line 105), without updating any internal thread or lock count fields of the `RecursiveLock`.

Happy to discuss or test some examples. My own use case works much better with the generic cond logic, and this seems like a safe fix to me given that the generic logic is well-tested and this seems not to be.

If you like the PR I'll send another one for the SDL2 branch.