Removes deprecated MDAnalysis.units.N_Avogadro

[IAlibay]

Related to #2230 ?

Changes made in this Pull Request:

• Removes MDAnalysis.units.N_Avogadro which was deprecated in 0.9

PR Checklist

• Tests? N/A
• Docs? N/A
• CHANGELOG updated?
• Issue raised/referenced? N/A?

[orbeckst]

lgtm - less code is good!

[codecov]

Codecov Report

Merging #2737 into develop will decrease coverage by 0.00%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #2737      +/-   ##
===========================================
- Coverage    91.31%   91.31%   -0.01%     
===========================================
  Files          176      176              
  Lines        24019    24018       -1     
  Branches      3160     3160              
===========================================
- Hits         21932    21931       -1     
  Misses        1459     1459              
  Partials       628      628              

Impacted Files	Coverage Δ
package/MDAnalysis/units.py	100.00% <ø> (ø)

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