Remove postcodes from Admin user search

[jsharkey13]

If postcode filtering was used with no other search terms, every single user was loaded from the database and we then attempted to filter them. This is not sustainable, and indeed did not work reliably. We can better filter users offline if we need location-based subsets.

This also allows us to remove all the apparatus for finding the lat-lon of postcodes we did not already have, too.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 40.12%. Comparing base (31ef070) to head (14dc092).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
...n/java/uk/ac/cam/cl/dtg/segue/api/AdminFacade.java	0.00%	1 Missing ⚠️
...va/uk/ac/cam/cl/dtg/segue/dao/LocationManager.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #784      +/-   ##
==========================================
- Coverage   40.28%   40.12%   -0.16%     
==========================================
  Files         546      543       -3     
  Lines       23794    23571     -223     
  Branches     2904     2867      -37     
==========================================
- Hits         9585     9458     -127     
+ Misses      13309    13229      -80     
+ Partials      900      884      -16     

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