Remove unused flatten_one_level function

[eric-wieser]

This has nothing to do with differential operators, and isn't used or documented.

In the very unlikely event that anyone was using this, the builtin itertools.chain.from_iterable provides similar functionality.

[codecov]

Codecov Report

Merging #175 into master will decrease coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #175      +/-   ##
==========================================
- Coverage   67.84%   67.78%   -0.07%     
==========================================
  Files           9        9              
  Lines        4684     4681       -3     
==========================================
- Hits         3178     3173       -5     
- Misses       1506     1508       +2

Impacted Files	Coverage Δ
galgebra/mv.py	64.54% <ø> (-0.11%)	⬇️
galgebra/utils.py	65.62% <0%> (-6.25%)	⬇️
galgebra/ga.py	73.5% <0%> (+0.06%)	⬆️

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