Remove nullable type handling for ARIMA and Exponential Smoothing

[eccabay]

Closes #4016

Sktime released support for nullable types in version 0.17.0

[codecov]

Codecov Report

Merging #4129 (4ad08c1) into main (535cbd5) will increase coverage by 40.0%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##            main   #4129      +/-   ##
========================================
+ Coverage   59.8%   99.7%   +40.0%     
========================================
  Files        349     349              
  Lines      37749   37704      -45     
========================================
+ Hits       22550   37587   +15037     
+ Misses     15199     117   -15082     

Impacted Files	Coverage Δ
...omponents/estimators/regressors/arima_regressor.py	100.0% <ø> (+20.2%)	⬆️
...tors/regressors/exponential_smoothing_regressor.py	100.0% <ø> (+21.6%)	⬆️
...alml/tests/component_tests/test_arima_regressor.py	100.0% <ø> (+100.0%)	⬆️
...nent_tests/test_exponential_smoothing_regressor.py	100.0% <ø> (+100.0%)	⬆️

... and 204 files with indirect coverage changes

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[eccabay]

Blocked on sktime conda - conda-forge/sktime-feedstock#70

[fkiraly]

looking into it, sorry for the delay

[jeremyliweishih]

LGTM

[fkiraly]

🎉 😃