Remove query constraints for /links-endpoint

[CasperWA]

According to Materials-Consortia/OPTIMADE#217 there is no reason why we cannot apply the full range of query parameters for the /links-endpoint.

This PR removes the constraints set on the query parameters for the /links-endpoint and as a bonus I found an elif that had a note, which said it could be removed after upgrading to pydantic v1, so I removed it... Looking around the code, I can't see why I shouldn't be able to do so.

[codecov]

Codecov Report

Merging #244 into master will increase coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #244      +/-   ##
==========================================
+ Coverage   87.53%   87.62%   +0.09%     
==========================================
  Files          45       45              
  Lines        1941     1932       -9     
==========================================
- Hits         1699     1693       -6     
+ Misses        242      239       -3     

Flag	Coverage Δ
#unittests	87.62% <ø> (+0.09%)	⬆️

Impacted Files	Coverage Δ
optimade/server/routers/links.py	100.00% <ø> (+10.00%)	⬆️
optimade/server/routers/utils.py	96.94% <ø> (+0.70%)	⬆️

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[ml-evs]

LGTM! Nice that we're getting to the stage where we can remove code :)