This commit harmonizes the seeding procedure of generators based
on o2::eventgen::GeneratorPythia8, following a development done in O2
(AliceO2Group/AliceO2#13281)

In principle, o2::eventgen::GeneratorPythia8 now performs it's own
seeding within the Init function. This seeding makes sure
to integrate with the `--seed` command line option of our event generation
executable.

The commit:
* Removes seeding code in O2DPG when it would override the default
  behaviour from O2.

* Uses a new dedicated function `setInitialSeed` instead of manipulating
  Pythia8 with config strings (in situations in which the user
  targets seeding based on ALIEN_PROC_ID for instance).

* Ensures that event generation becomes more repeatable in situations
  when the same `--seed` is given to the event generation executable.
  This may help to debug crashes on the GRID etc.