Fix Env Var item deletion

[Steve-Mcl]

closes #4067

Description

Fixes env Var deletion

Related Issue(s)

#4067

Checklist

• I have read the contribution guidelines
• Suitable unit/system level tests have been added and they pass
• Documentation has been updated
  • Upgrade instructions
  • Configuration details
  • Concepts
• Changes flowforge.yml?
  • Issue/PR raised on FlowFuse/helm to update ConfigMap Template
  • Issue/PR raised on FlowFuse/CloudProject to update values for Staging/Production

Labels

• Includes a DB migration? -> add the area:migration label

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.72%. Comparing base (bfbe907) to head (2c3419f).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4072   +/-   ##
=======================================
  Coverage   78.72%   78.72%           
=======================================
  Files         284      284           
  Lines       13009    13009           
  Branches     2897     2897           
=======================================
  Hits        10241    10241           
  Misses       2768     2768           

Flag	Coverage Δ
backend	78.72% <ø> (ø)

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