Fix delete station (fixes #492)

[Shrhawk]

Checklist:

• [ ] All tests are passing
• [ ] I have added any necessary documentation and/or release notes
• [ ] I have followed the commit message guidelines and the "before submitting a pull request" guidelines
• [ ] If I introduced (or abolished) any third-party library, I will write a short paragraph in this PR according to the guidelines
• [ ] I have removed all obsoleted code

[Shrhawk]

Related pull request https://github.com/openmeteo/enhydris-synoptic/pull/81

[codecov[bot]]

Codecov Report

Merging #496 (84ef13c) into master (e6d7509) will decrease coverage by 0.11%. The diff coverage is 97.67%.

@@            Coverage Diff             @@
##           master     #496      +/-   ##
==========================================
- Coverage   98.30%   98.20%   -0.11%     
==========================================
  Files          41       41              
  Lines        2127     2119       -8     
==========================================
- Hits         2091     2081      -10     
- Misses         36       38       +2     

Impacted Files	Coverage Δ
enhydris/admin/__init__.py	96.42% <ø> (-0.35%)	:arrow_down:
enhydris/api/csv.py	100.00% <ø> (ø)
enhydris/api/permissions.py	100.00% <ø> (+5.00%)	:arrow_up:
enhydris/api/serializers.py	95.60% <ø> (-0.19%)	:arrow_down:
enhydris/api/urls.py	100.00% <ø> (ø)
enhydris/models/timeseries_group.py	93.40% <ø> (-1.80%)	:arrow_down:
enhydris/telemetry/forms.py	100.00% <ø> (ø)
enhydris/telemetry/views.py	98.31% <ø> (ø)
enhydris/admin/station.py	97.95% <92.85%> (-0.42%)	:arrow_down:
enhydris/api/views.py	97.89% <95.83%> (-0.37%)	:arrow_down:
... and 7 more

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