slice: don't overwrite sliceId/tileId at start of each slice

This commit fixes an error whereby current the slice and tile ids
would be reset to zero at the start of each geometry slice, causing the
slice and tile ids set by the partitioning method to be overwritten.

tmc3/encoder.cpp

@@ -236,9 +236,6 @@ PCCTMC3Encoder3::compressPartition(
   quantization(inputPointCloud);
 
   // geometry encoding
-  _sliceId = 0;
-  _tileId = 0;
-
   if (1) {
     PayloadBuffer payload(PayloadType::kGeometryBrick);