BUG: unlock annotate_timestamp callback for polar and spherical geometries

[neutrinoceros]

PR Summary

This callback already works as intended with simple parametrization, I think it's worth unlocking now.

import yt
from yt.testing import fake_amr_ds

ds = fake_amr_ds(geometry="spherical")
p = yt.SlicePlot(ds, "r", "Density")
p.annotate_timestamp(draw_inset_box=True)
p.save("/tmp/sph.png")