Don't clear channels on writer close() (#1194)

Don't clear channels on writer close(). This matches the behavior
schemas has, allowing for stable ids after closing and opening a new
file with the same reader. See
https://foxglove.slack.com/archives/C02H1JXG3C3/p1720827358364829?thread_ts=1720825408.044619&cid=C02H1JXG3C3

Alternatives to this involve more bookkeeping on the recorder
implementation side to re-register all schemas/channels on split.

cpp/mcap/include/mcap/writer.inl

@@ -473,7 +473,6 @@ void McapWriter::terminate() {
   zstdChunk_.reset();
 #endif
 
-  channels_.clear();
   attachmentIndex_.clear();
   metadataIndex_.clear();
   chunkIndex_.clear();