Remove function since_until_to_time_range

[mistercrunch]

@betodealmeida I had missed removing this function.

It's taken care of earlier in the pipeline with legacy.update_time_range.

[betodealmeida]

+1

[codecov-io]

Codecov Report

Merging #5560 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5560      +/-   ##
==========================================
- Coverage   63.25%   63.22%   -0.03%     
==========================================
  Files         351      351              
  Lines       22258    22251       -7     
  Branches     2470     2470              
==========================================
- Hits        14079    14068      -11     
- Misses       8164     8168       +4     
  Partials       15       15

Impacted Files	Coverage Δ
superset/utils.py	88.88% <ø> (-0.31%)	⬇️
superset/viz.py	79.34% <ø> (-0.02%)	⬇️
superset/models/core.py	86.57% <0%> (-0.52%)	⬇️

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