Remove coord discontinuity check in regrid

[sloosvel]

Description

This PR removes the discontinuity check on the grid because iris.util.find_discontiguities realises the auxiliary coordinates
Closes #2117
Link to documentation:

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[codecov]

Codecov Report

Merging #2119 (58a61c4) into main (1101d36) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #2119      +/-   ##
==========================================
- Coverage   93.09%   93.09%   -0.01%     
==========================================
  Files         237      237              
  Lines       12806    12794      -12     
==========================================
- Hits        11922    11910      -12     
  Misses        884      884              

Impacted Files	Coverage Δ
esmvalcore/preprocessor/_regrid.py	98.25% <ø> (-0.06%)	⬇️