MSC3938: Remove {keyID} from key lookups over federation

[S7evinK]

As per MSC3938, which landed in Matrix 1.6 (so this is part of #3224 )

No tests, as this basically only removes code.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 58.82353% with 7 lines in your changes missing coverage. Please review.

Project coverage is 49.35%. Comparing base (add73ec) to head (2be159f).

Files with missing lines	Patch %	Lines
federationapi/routing/routing.go	16.66%	4 Missing and 1 partial ⚠️
federationapi/internal/query.go	66.66%	1 Missing ⚠️
test/memory_federation_db.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3472      +/-   ##
==========================================
- Coverage   49.39%   49.35%   -0.05%     
==========================================
  Files         524      524              
  Lines       59737    59714      -23     
==========================================
- Hits        29507    29470      -37     
- Misses      26748    26756       +8     
- Partials     3482     3488       +6     

Flag	Coverage Δ
unittests	49.35% <58.82%> (-0.05%)	⬇️

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[S7evinK]

Failing Sytest makes me sad. :(