FullGroup selection keyword removal

[IAlibay]

Completes #268

Changes made in this Pull Request:

• Removes the fullgroup keyword from selections (and related test).

PR Checklist

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

[IAlibay]

So the test removal wasn't necessary since it was just testing that an error would occur, so if we want I can always add it back in.

[codecov]

Codecov Report

Merging #2500 into develop will increase coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #2500      +/-   ##
===========================================
+ Coverage    90.25%   90.25%   +<.01%     
===========================================
  Files          169      169              
  Lines        23225    23217       -8     
  Branches      3009     3010       +1     
===========================================
- Hits         20962    20955       -7     
+ Misses        1656     1655       -1     
  Partials       607      607

Impacted Files	Coverage Δ
package/MDAnalysis/core/groups.py	98.24% <ø> (ø)	⬆️
package/MDAnalysis/core/selection.py	99.48% <ø> (+0.15%)	⬆️
package/MDAnalysis/analysis/density.py	65.45% <0%> (+0.5%)	⬆️

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