Optimize getExtensionClass method.

[carryxyh]

Optimize getExtensionClass method.
Just return the class instead of throwing exception.

org.apache.dubbo.common.extension.ExtensionLoader#getExtensionClass just has one caller that is hasExtension. So it will not effect other logic.

[codecov-io]

Codecov Report

Merging #2788 into master will decrease coverage by 0.11%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2788      +/-   ##
==========================================
- Coverage    63.6%   63.49%   -0.12%     
==========================================
  Files         577      577              
  Lines       25935    25930       -5     
  Branches     4544     4544              
==========================================
- Hits        16495    16463      -32     
- Misses       7283     7302      +19     
- Partials     2157     2165       +8

Impacted Files	Coverage Δ
...apache/dubbo/common/extension/ExtensionLoader.java	80.44% <100%> (-0.2%)	⬇️
...ache/dubbo/remoting/transport/AbstractChannel.java	75% <0%> (-12.5%)	⬇️
...he/dubbo/remoting/transport/netty/NettyClient.java	72.88% <0%> (-8.48%)	⬇️
...e/dubbo/remoting/transport/netty/NettyChannel.java	54.11% <0%> (-8.24%)	⬇️
...org/apache/dubbo/rpc/filter/ActiveLimitFilter.java	83.33% <0%> (-5.56%)	⬇️
...he/dubbo/remoting/transport/netty/NettyServer.java	67.85% <0%> (-3.58%)	⬇️
...c/main/java/org/apache/dubbo/rpc/RpcException.java	85.71% <0%> (-3.58%)	⬇️
...pache/dubbo/remoting/transport/AbstractClient.java	65.94% <0%> (-2.17%)	⬇️
...e/dubbo/registry/integration/RegistryProtocol.java	81.25% <0%> (-1.93%)	⬇️
...ache/dubbo/remoting/transport/mina/MinaClient.java	56.92% <0%> (-1.54%)	⬇️

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[carryxyh]

@beiwei30
Simple pr. Would u pls give a review?

[ralf0131]

LGTM.