[Fixes #12921] Fix Layers admin interface raise error

[mattiagiupponi]

ref #12921

The new metadata editor require an id while calling the autocomplete endpoint, so we cannot switch to it.
It means that we can mantain the tkeywords in the admin interface, but without autocomplete (like we have for regions and other metadata):

Checklist

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.94%. Comparing base (267ad61) to head (cc0cf35).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #12922      +/-   ##
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+ Coverage   61.92%   67.94%   +6.02%     
==========================================
  Files         978      978              
  Lines       59096    59091       -5     
  Branches     6907     6907              
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+ Hits        36593    40150    +3557     
+ Misses      20929    17296    -3633     
- Partials     1574     1645      +71     

[giohappy]

@mattiagiupponi let's remove this field completely from the admin.