Remove unused and untested muon fitting code

[maxnoe]

First step in refactoring of the muon code.

This removed a bunch of unused and untested functions, added by myself four years ago.

Maybe I'll add some of these back, but then by integrating it with the other code and with tests added.

So for now, let's just purge this.

[kosack]

If it's not being used elsewhere, I'm ok with it.

[maxnoe]

At least not in the LST muon code

[maxnoe]

Which I'm going to refactor as well along with this...

[rlopezcoto]

isn't this efficiency_fit the one used to calculate the muon_efficiency given as the output of the muon code?

[maxnoe]

No, this is a different, untested function doing the same thing.

[kosack]

looks like you need to remove vacanti94 from bibliography.rst. It's somewhat annoying that you need to use all references, or they cause an error, but I don't see a way to disable that.

/home/travis/build/cta-observatory/ctapipe/docs/bibliography.rst:13:Citation [vacanti94] is not referenced.
Error: Error building sphinx doc

Or alternately, mention this reference somewhere.

[codecov]

Codecov Report

Merging #1254 into master will increase coverage by 0.51%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1254      +/-   ##
==========================================
+ Coverage   86.78%   87.30%   +0.51%     
==========================================
  Files         192      192              
  Lines       11967    12129     +162     
==========================================
+ Hits        10386    10589     +203     
+ Misses       1581     1540      -41     

Impacted Files	Coverage Δ
ctapipe/image/muon/fitting.py	100.00% <ø> (+45.36%)	⬆️
ctapipe/conftest.py	100.00% <0.00%> (ø)
ctapipe/io/sources.py	0.00% <0.00%> (ø)
ctapipe/image/reducer.py	100.00% <0.00%> (ø)
ctapipe/image/toymodel.py	100.00% <0.00%> (ø)
ctapipe/coordinates/__init__.py	100.00% <0.00%> (ø)
ctapipe/core/tests/test_traits.py	100.00% <0.00%> (ø)
ctapipe/coordinates/ground_frames.py	100.00% <0.00%> (ø)
ctapipe/image/tests/test_extractor.py	100.00% <0.00%> (ø)
ctapipe/coordinates/telescope_frame.py	100.00% <0.00%> (ø)
... and 48 more

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[maxnoe]

@kosack could you re-approve