front: remove grid margins

[eckter]

They were redundant with the "green signal" requirements of our conflict detection module, we'd miss out on solutions for no good reason.

Fix #11020

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 80.62%. Comparing base (164c697) to head (1b57b83).
Report is 1 commits behind head on dev.

Files with missing lines	Patch %	Lines
front/src/reducers/osrdconf/stdcmConf/index.ts	50.00%	2 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##              dev   #11044      +/-   ##
==========================================
- Coverage   80.63%   80.62%   -0.01%     
==========================================
  Files        1099     1101       +2     
  Lines      112187   112322     +135     
  Branches      745      749       +4     
==========================================
+ Hits        90460    90565     +105     
- Misses      21684    21714      +30     
  Partials       43       43              

Flag	Coverage Δ
editoast	72.18% <ø> (-0.01%)	⬇️
front	89.92% <50.00%> (-0.03%)	⬇️
gateway	2.18% <ø> (ø)
osrdyne	2.53% <ø> (ø)
railjson_generator	87.58% <ø> (ø)
tests	87.90% <ø> (ø)

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[clarani]

LGTM ✅