refactor: remove unused api in cluster manager

[neverchanje]

What's changed and what's your intention?

• Removed deactivate_worker_node in ClusterManager.

[codecov]

Codecov Report

Merging #2139 (67ebf1a) into main (8191749) will increase coverage by 0.06%.
The diff coverage is 94.66%.

@@            Coverage Diff             @@
##             main    #2139      +/-   ##
==========================================
+ Coverage   70.53%   70.59%   +0.06%     
==========================================
  Files         645      645              
  Lines       81657    81673      +16     
==========================================
+ Hits        57595    57659      +64     
+ Misses      24062    24014      -48     

Flag	Coverage Δ
rust	70.59% <94.66%> (+0.06%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/frontend/src/session.rs	51.03% <ø> (ø)
src/meta/src/barrier/mod.rs	69.00% <ø> (ø)
src/meta/src/rpc/service/ddl_service.rs	0.00% <ø> (ø)
src/meta/src/cluster/mod.rs	59.65% <90.00%> (+2.77%)	⬆️
src/frontend/src/scheduler/worker_node_manager.rs	69.86% <100.00%> (+51.44%)	⬆️
.../src/executor/managed_state/aggregation/extreme.rs	90.02% <0.00%> (-0.27%)	⬇️
src/storage/src/hummock/local_version_manager.rs	91.05% <0.00%> (+2.03%)	⬆️
src/meta/src/hummock/mock_hummock_meta_client.rs	49.36% <0.00%> (+5.06%)	⬆️
src/common/src/util/addr.rs	95.69% <0.00%> (+6.45%)	⬆️
... and 1 more

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[yezizp2012]

get_cluster_core_guard is used for snapshot consistency when synced to frontend. Suggest to keep it. Cc @HuaHuaY