Remove systemd config on relation departed

[Thanhphan1147]

When agent relation is removed the systemd config file is not removed, we should clean it up after stopping the service

Checklist

• The charm style guide was applied
• The contributing guide was applied
• The changes are compliant with ISD054 - Managing Charm Complexity
• The documentation is generated using src-docs
• The documentation for charmhub is updated
• The PR is tagged with appropriate label (urgent, trivial, complex)

[github-actions]

Test coverage for 4e53cdf

Name                    Stmts   Miss Branch BrPart  Cover   Missing
-------------------------------------------------------------------
src/agent_observer.py      42      0      4      0   100%
src/charm.py               53      0      6      0   100%
src/charm_state.py         59      4      8      1    93%   114, 172-174
src/service.py             95      2     14      5    94%   106->118, 133, 165, 188->193, 191->188
-------------------------------------------------------------------
TOTAL                     249      6     32      6    96%

Static code analysis report

Run started:2024-02-14 08:44:40.614359

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 1062
  Total lines skipped (#nosec): 0
  Total potential issues skipped due to specifically being disabled (e.g., #nosec BXXX): 0

Run metrics:
  Total issues (by severity):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
  Total issues (by confidence):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
Files skipped (0):