fix(abstractField): clearEncodingFunctions()

[gbossert]

The clearEncodingFunctions() doesn't work properly. It prevented multiple consecutive calls to encodingFunctions setter.

[codecov-io]

Codecov Report

Merging #115 into next will decrease coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##             next     #115      +/-   ##
==========================================
- Coverage   68.57%   68.53%   -0.05%     
==========================================
  Files         157      157              
  Lines        9483     9483              
==========================================
- Hits         6503     6499       -4     
- Misses       2980     2984       +4

Impacted Files	Coverage Δ
...unctions/EncodingFunctions/TypeEncodingFunction.py	85.45% <ø> (ø)	⬆️
...etzob/src/netzob/Model/Vocabulary/AbstractField.py	75.1% <100%> (ø)	⬆️
...Model/Grammar/Transitions/OpenChannelTransition.py	79.31% <0%> (-13.8%)	⬇️

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