Remove `player_id` argument in `Env.observe`

[sotetsuk]

While this feature is rarely used, it makes the implementation complicated.

• [ ] We first make this argument Optional and =None as default, then
• [ ] Add warning to users for a while (playerd_id is not None is deprecated)
• [ ] If no one complains about it, we'll remove it and release as 3.0.0

import jax
import warnings

@jax.jit
def func(x=None):
    if x is not None:
        warnings.warn("This feature is deprecated", DeprecationWarning)
    return x

if __name__ == "__main__":
    print(func(3))
    print(func(2))
    print(func(4))
    print(func(5))