Remove hacks to avoid previous aesara gammainc(c) limitations

[ricardoV94]

This is a reminder issue

The gamma functions in aesara were changed to work with invalid values (returning nan) instead of doing a c-level assertion (pymc-devs/pytensor#290). The following lines can be changed / removed once pymc3 is paired with the next version of aesara:

Distributions:

https://github.com/pymc-devs/pymc3/blob/89916ad724ae88ebb4c1af3d65919b87b318f180/pymc3/distributions/continuous.py#L2631-L2634

https://github.com/pymc-devs/pymc3/blob/89916ad724ae88ebb4c1af3d65919b87b318f180/pymc3/distributions/continuous.py#L2805-L2808

Unittests:

https://github.com/pymc-devs/pymc3/blob/e40304b98503907f620d67d4a9ff6e42c63171d1/pymc3/tests/test_distributions.py#L1231-L1240

https://github.com/pymc-devs/pymc3/blob/e40304b98503907f620d67d4a9ff6e42c63171d1/pymc3/tests/test_distributions.py#L1253-L1262