Remove wall-time / unused skip tick protection (#11412)

Spigot still maintains some partial implementation of "tick skipping", a
practice in which the MinecraftServer.currentTick field is updated not
by an increment of one per actual tick, but instead set to
System.currentTimeMillis() / 50. This behaviour means that the tracked
tick may "skip" a tick value in case a previous tick took more than the
expected 50ms.

To compensate for this in important paths, spigot/craftbukkit
implements "wall-time". Instead of incrementing/decrementing ticks on
block entities/entities by one for each call to their tick() method,
they instead increment/decrement important values, like
an ItemEntity's age or pickupDelay, by the difference of
`currentTick - lastTick`, where `lastTick` is the value of
`currentTick` during the last tick() call.

These "fixes" however do not play nicely with minecraft's simulation
distance as entities/block entities implementing the above behaviour
would "catch up" their values when moving from a non-ticking chunk to a
ticking one as their `lastTick` value remains stuck on the last tick in
a ticking chunk and hence lead to a large "catch up" once ticked again.

Paper completely removes the "tick skipping" behaviour (See patch
"Further-improve-server-tick-loop"), making the above precautions
completely unnecessary, which also rids paper of the previous described
incompatibility with non-ticking chunks.

patches/server/0390-Optimize-NetworkManager-Exception-Handling.patch

@@ -1,23 +0,0 @@
-From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
-From: Andrew Steinborn <git@steinborn.me>
-Date: Sun, 5 Jul 2020 22:38:18 -0400
-Subject: [PATCH] Optimize NetworkManager Exception Handling
-
-
-diff --git a/src/main/java/net/minecraft/network/Varint21FrameDecoder.java b/src/main/java/net/minecraft/network/Varint21FrameDecoder.java
-index 421dd76816063d56ea80339b77531729edd6aa55..1523d69b7b332f0085f40310a94d406da6513edc 100644
---- a/src/main/java/net/minecraft/network/Varint21FrameDecoder.java
-+++ b/src/main/java/net/minecraft/network/Varint21FrameDecoder.java
-@@ -39,6 +39,12 @@ public class Varint21FrameDecoder extends ByteToMessageDecoder {
-     }
- 
-     protected void decode(ChannelHandlerContext channelHandlerContext, ByteBuf byteBuf, List<Object> list) {
-+        // Paper start - Perf: Optimize exception handling; if channel is not active just discard the packet
-+        if (!channelHandlerContext.channel().isActive()) {
-+            byteBuf.skipBytes(byteBuf.readableBytes());
-+            return;
-+        }
-+        // Paper end - Perf: Optimize exception handling
-         byteBuf.markReaderIndex();
-         this.helperBuf.clear();
-         if (!copyVarint(byteBuf, this.helperBuf)) {