MINOR: [Python] Remove obsolete is_map TODO comment (#48353)

### Rationale for this change

This TODO was added in eaeb5d4. However, now we have map support properly, and its test was added in e0c1ffe

### What changes are included in this PR?

This PR proposes to remove obsolete is_map TODO comment.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.

Authored-by: Hyukjin Kwon <[email protected]>
Signed-off-by: AlenkaF <[email protected]>

Author: HyukjinKwon

python/pyarrow/tests/test_types.py

@@ -260,9 +260,6 @@ def test_is_run_end_encoded():
     assert not types.is_run_end_encoded(pa.utf8())
 
 
-# TODO(wesm): is_map, once implemented
-
-
 def test_is_binary_string():
     assert types.is_binary(pa.binary())
     assert not types.is_binary(pa.string())