fix: Remove trial_id check when loading Optuna study

[rclarsfull]

Description

If no trail id ist given to load _trail() it, will load the best trail. When checking trai_id != None this disables this feature and makes it unusabel.

Motivation and Context

I just deletet the check, so its possile to just do this to train with the best taril.

python rl_zoo3/train.py \
  --algo ppo \
  --env MergeEnv-v2 \
  --storage sqlite:///logs/mergeenv_optuna.db \
  --study-name MergeEventOptimizev1 \
  -P 

Issue: #498

closes #498

• I have raised an issue to propose this change (required for new features and bug fixes)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)

Checklist:

• I've read the CONTRIBUTION guide (required)
• I have updated the changelog accordingly (required).
• My change requires a change to the documentation.
• I have updated the tests accordingly (required for a bug fix or a new feature).
• I have updated the documentation accordingly.
• I have reformatted the code using make format (required)
• I have checked the codestyle using make check-codestyle and make lint (required)
• I have ensured make pytest and make type both pass. (required)

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[araffin]

see line just above

[araffin]

I think this check is needed (see failures on CI), instead we could allow passing a special value like -1 to allow loading the best trial