Remove mentioning of tostring method (#2309)

`numpy.ndarray.tostring` method was deprecated since `1.19` and is going
to be removed in `2.3.0` release.

This PR proposes to remove mentioning of `tostring` method in a list of
rendered documentation and as TODO method for implementation in
`dpnp.ndarray`.

Author: antonwolfy

doc/reference/ndarray.rst

@@ -134,7 +134,6 @@ Array conversion
 
    dpnp.ndarray.item
    dpnp.ndarray.tolist
-   dpnp.ndarray.tostring
    dpnp.ndarray.tobytes
    dpnp.ndarray.tofile
    dpnp.ndarray.dump

dpnp/dpnp_array.py

@@ -1860,7 +1860,6 @@ def to_device(self, device, /, *, stream=None):
     # 'tobytes',
     # 'tofile',
     # 'tolist',
-    # 'tostring',
 
     def trace(self, offset=0, axis1=0, axis2=1, dtype=None, out=None):
         """