Remove postcodes from User Manager

[jsharkey13]

This goes alongside isaacphysics/isaac-api#784; postcode search was unreliable and no longer fit for purpose.

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 43.72%. Comparing base (db9f57e) to head (a38b700).
⚠️ Report is 28 commits behind head on main.

Files with missing lines	Patch %	Lines
src/app/components/pages/AdminUserManager.tsx	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2129      +/-   ##
==========================================
- Coverage   43.72%   43.72%   -0.01%     
==========================================
  Files         584      584              
  Lines       24767    24765       -2     
  Branches     7350     8244     +894     
==========================================
- Hits        10830    10829       -1     
+ Misses      13887    13879       -8     
- Partials       50       57       +7     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.