[c++] remove unused variable 'need_connect_cnt'

[jameslamb]

clang++ complains about this variable in linkers_socket.cpp being unused.

.../src/network/linkers_socket.cpp:174:7: warning: variable 'need_connect_cnt' set but not used [-Wunused-but-set-variable]
  int need_connect_cnt = 0;

That looks correct to me, based on the output of this:

git grep need_connect_cnt

Seems that that variable is being incremented but not used for anything. I suspect that's left over from some previous refactoring.

This proposes removing that unused variable.

[github-actions]

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