Revert "Don't flush packet queue off main thread"

This reverts commit e3105bc57a.

Sadly not gonna work due to server pings

Spigot-Server-Patches/Handle-Excessive-Signs-in-Chunks-creating-large-pack.patch

@@ -14,7 +14,7 @@ With further contributions from
 Co-Authored-by: Spottedleaf <spottedleaf@spottedleaf.dev>
 
 diff --git a/src/main/java/net/minecraft/server/NetworkManager.java b/src/main/java/net/minecraft/server/NetworkManager.java
-index b1dededc15..07e3ddb6e9 100644
+index b1dededc15..e966b6edcc 100644
 --- a/src/main/java/net/minecraft/server/NetworkManager.java
 +++ b/src/main/java/net/minecraft/server/NetworkManager.java
 @@ -0,0 +0,0 @@ public class NetworkManager extends SimpleChannelInboundHandler<Packet<?>> {
@@ -73,7 +73,7 @@ index b1dededc15..07e3ddb6e9 100644
 +
 +            this.packetQueue.addAll(packets); // atomic
 +        }
-+        if (MCUtil.isMainThread()) this.sendPacketQueue();
++        this.sendPacketQueue(); // ensure only one flush call is made
 +        // Paper end
      }