removed funcsigs (not needed for Py >= 3.6)

[orbeckst]

Changes made in this Pull Request:

• removed funcsigs that were introduced in PR Always transplant a stub for transplanted methods #2427 as a compatibility layer (needed for backport PR backport of stub transplants of AtomGroup (PR #2427) #3007)
• not needed for 2.x (Python ≥ 3.6)

PR Checklist

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

[codecov]

Codecov Report

Merging #3008 into develop will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #3008      +/-   ##
===========================================
+ Coverage    93.05%   93.12%   +0.07%     
===========================================
  Files          186      171      -15     
  Lines        24663    22652    -2011     
  Branches      3197     3197              
===========================================
- Hits         22950    21095    -1855     
+ Misses        1665     1509     -156     
  Partials        48       48              

Impacted Files	Coverage Δ
package/MDAnalysis/core/topologyattrs.py	96.61% <100.00%> (+0.17%)	⬆️
lib/__init__.py
topology/base.py
__init__.py
topology/__init__.py
coordinates/base.py
util.py
transformations/__init__.py
formats/__init__.py
core/__init__.py
... and 6 more

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[tylerjereddy]

+1 straightforward, thanks!