Removed 3 deprecated methods.

[MichaelClerx]

Closes #734

[codecov]

Codecov Report

Merging #737 (e017368) into main (4578fe6) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##              main      #737   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          102       102           
  Lines        19254     19242   -12     
=========================================
- Hits         19254     19242   -12     

Impacted Files	Coverage Δ
myokit/_model_api.py	100.00% <ø> (ø)
myokit/_protocol.py	100.00% <ø> (ø)

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