Remove optional feet position in DummyWalkingPatternGenerator

include/sot/talos_balance/dummy-walking-pattern-generator.hh

...n/sot_talos_balance/test/appli_dcmZmpControl_estimator.py

python/sot_talos_balance/test/appli_no_admittance.py

...on/sot_talos_balance/test/test_dcmZmpControl_estimator.py

python/sot_talos_balance/test/test_no_admittance.py

src/dummy-walking-pattern-generator.cpp