fix(emergency_handler): stop using heartbeat_checher to make subscriber unique

[h-ohta]

Description

Now, subscriber for hazard_status is duplicate due to processing data and checking heartbeat.
I think a subscriber should be as unique as possible for a topic.
I'll fix it.
Moreover, I'll delete heartbeat checker implementation because it is only used in emergency_handler

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[codecov]

Codecov Report

Base: 10.76% // Head: 10.75% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (a2de17b) compared to base (253b5b9).
Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2138      +/-   ##
==========================================
- Coverage   10.76%   10.75%   -0.01%     
==========================================
  Files        1186     1186              
  Lines       84749    84822      +73     
  Branches    19827    19827              
==========================================
  Hits         9119     9119              
- Misses      65991    66064      +73     
  Partials     9639     9639              

Flag	Coverage Δ		*Carryforward flag
differential	12.36% <0.00%> (?)
total	10.73% <0.00%> (ø)		Carriedforward from 253b5b9

*This pull request uses carry forward flags. Click here to find out more.

Impacted Files	Coverage Δ
...clude/emergency_handler/emergency_handler_core.hpp	0.00% <ø> (ø)
...r/src/emergency_handler/emergency_handler_core.cpp	0.00% <0.00%> (ø)

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[kenji-miyake]

This is a trade-off between simplicity and computation costs (this time, creating multiple callbacks).
If you will maintain this package, this change is okay for me.

[h-ohta]

I'ts true, but deleting and integrating heart beat library is more simple because the library is only used at only one node now.

[h-ohta]

FYI #1501 (comment)