Remove ancient deprecated functions related to specifying observables/noise models inside SBML

[dweindl]

They are obsolete since the introduction of the observable table.

[codecov-commenter]

Codecov Report

Merging #140 (ae55288) into develop (93d7f60) will increase coverage by 0.41%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #140      +/-   ##
===========================================
+ Coverage    77.30%   77.72%   +0.41%     
===========================================
  Files           27       27              
  Lines         2956     2904      -52     
  Branches       724      718       -6     
===========================================
- Hits          2285     2257      -28     
+ Misses         488      468      -20     
+ Partials       183      179       -4     

Impacted Files	Coverage Δ
petab/core.py	87.90% <ø> (+0.88%)	⬆️
petab/sbml.py	63.15% <100.00%> (+3.40%)	⬆️

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[dweindl]

• Simplify test model creation - use simplesbml #139