fix(net_monitor): fix cppcheck warnings

[veqcc]

Description

This is a fix based on cppcheck warnings below

system/system_monitor/src/net_monitor/net_monitor.cpp:285:12: style: The scope of the variable 'unit_reassembles_failed' can be reduced. [variableScope]
  uint64_t unit_reassembles_failed = 0;
           ^
system/system_monitor/src/net_monitor/net_monitor.cpp:159:15: style: Unused variable: error_str [unusedVariable]
  std::string error_str;
              ^
system/system_monitor/src/net_monitor/net_monitor.cpp:160:28: style: Unused variable: interface_names [unusedVariable]
  std::vector<std::string> interface_names;
                           ^
system/system_monitor/src/net_monitor/net_monitor.cpp:457:21: style: Variable 'ether_request.cmd' is assigned a value that is never used. [unreadVariable]
  ether_request.cmd = ETHTOOL_GSET;
                    ^

Tests performed

Not applicable.

Effects on system behavior

Not applicable.

Interface changes

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 0.00%. Comparing base (507e3f4) to head (99b254c).
Report is 50 commits behind head on main.

Files	Patch %	Lines
...tem/system_monitor/src/net_monitor/net_monitor.cpp	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #7573       +/-   ##
==========================================
- Coverage   14.84%   0.00%   -14.85%     
==========================================
  Files        1999      54     -1945     
  Lines      139163    4014   -135149     
  Branches    43716     440    -43276     
==========================================
- Hits        20661       0    -20661     
+ Misses      95731    4014    -91717     
+ Partials    22771       0    -22771     

Flag	Coverage Δ
differential	0.00% <0.00%> (?)
total	?

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[veqcc]

@ito-san @TetsuKawa
Could you approve it?

[ito-san]

@ito-san @TetsuKawa Could you approve it?

@veqcc checking if the corrections are working properly, so please wait.

[ito-san]

@veqcc
Verified it works correctly. Thanks!