Remove connections config of method level

[CrazyHZM]

What is the purpose of the change

Method config can no longer set connections, the definition in xsd can be deleted directly.
Fix #2769

Brief changelog

Verifying this change

Checklist

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[codecov-commenter]

Codecov Report

Merging #9011 (b089aef) into 3.0 (17f85db) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.0    #9011      +/-   ##
============================================
- Coverage     63.39%   63.38%   -0.01%     
+ Complexity      315      313       -2     
============================================
  Files          1188     1188              
  Lines         50447    50450       +3     
  Branches       7533     7533              
============================================
  Hits          31980    31980              
- Misses        14980    14985       +5     
+ Partials       3487     3485       -2     

Impacted Files	Coverage Δ
...e/dubbo/remoting/transport/netty/NettyChannel.java	52.27% <0.00%> (-7.96%)	⬇️
...vent/listener/ServiceInstancesChangedListener.java	77.11% <0.00%> (-1.28%)	⬇️
...ting/zookeeper/curator/CuratorZookeeperClient.java	70.38% <0.00%> (-0.98%)	⬇️
...ng/transport/dispatcher/all/AllChannelHandler.java	89.65% <0.00%> (ø)
...ng/zookeeper/curator5/Curator5ZookeeperClient.java	67.47% <0.00%> (ø)
...n/java/org/apache/dubbo/common/utils/NetUtils.java	69.03% <0.00%> (+0.32%)	⬆️
.../org/apache/dubbo/rpc/protocol/tri/TripleUtil.java	43.52% <0.00%> (+0.41%)	⬆️
...pache/dubbo/registry/support/AbstractRegistry.java	79.71% <0.00%> (+1.44%)	⬆️
.../apache/dubbo/remoting/transport/AbstractPeer.java	65.21% <0.00%> (+4.34%)	⬆️
...ng/exchange/support/header/HeartbeatTimerTask.java	73.68% <0.00%> (+5.26%)	⬆️
... and 1 more

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