fix: remove no longer relevant MaybeNodata type

now that `Nodata` is allowed to be `None`,
'MaybeNodata' is now confusing, removing it.

Let's assume that it never was a part of the
`odc.geo` API.

odc/geo/types.py

@@ -23,7 +23,6 @@
 MaybeFloat = Optional[float]
 FillValue = Union[float, int]
 Nodata = Union[float, int, None]
-MaybeNodata = Union[float, int, str, None]
 SomeNodata = Union[float, int, str, None, Literal["auto"]]
 T = TypeVar("T")
 T1 = TypeVar("T1")