deleteBlocks.php

<?php

$world = $this->getServer()->getDefaultLevel();
$radius = 136;
$total = (($radius * 2) + 1) ** 2;
$count = 0;
$bList = clone \pocketmine\block\Block::$list;

$search = [];

$replace = [];

for($i = 0; $i < 256; ++$i){
	if($bList[$i] === null){
		$search[] = chr($i);
		$replace[] = "\x00";
	}
}

$reflection = new \ReflectionClass(\pocketmine\level\format\generic\BaseFullChunk::class);
$blocks = $reflection->getProperty("blocks");
$blocks->setAccessible(true);

for($chunkX = -$radius; $chunkX <= $radius; ++$chunkX){
	for($chunkZ = -$radius; $chunkZ <= $radius; ++$chunkZ){
		$chunk = $world->getChunk($chunkX, $chunkZ, false);
		if($chunk !== null){
			$blocks->setValue($chunk, str_replace($search, $replace, $chunk->getBlockIdArray()));
			$chunk->setChanged();
			$world->setChunk($chunkX, $chunkZ, $chunk, false);
			$world->saveChunks();
		}
		$world->unloadChunk($chunkX, $chunkZ, false);
		++$count;
	}
	echo "[".round(($count/$total) * 100, 0)."%] $count/$total\n";
}
$world->save(true);

Thats the hackiest way i've ever seen.