Remove deprecated functions related to sbml-observables

[dweindl]

Removes obsolete and deprecated functions from when observables were defined inside SBML models and when there was no observable table.

[codecov-commenter]

Codecov Report

Merging #131 (5323379) into develop (684c2c2) will increase coverage by 0.36%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #131      +/-   ##
===========================================
+ Coverage    76.87%   77.24%   +0.36%     
===========================================
  Files           27       27              
  Lines         3005     2957      -48     
  Branches       731      723       -8     
===========================================
- Hits          2310     2284      -26     
+ Misses         509      490      -19     
+ Partials       186      183       -3     

Impacted Files	Coverage Δ
petab/measurements.py	90.12% <ø> (+10.12%)	⬆️
petab/problem.py	67.76% <ø> (+0.83%)	⬆️
petab/sbml.py	59.74% <100.00%> (-2.21%)	⬇️

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[FFroehlich]

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