GH-48355: [Python] Remove obsolete snprintf workaround for Python 3.9 (#48354)

### Rationale for this change

e9b9042 added the workaround for Python 3.9 on Windows. However, PyArrow dropped Python 3.9 at aedb752. Per python/cpython@e822e37, this should be fine to be removed.

### What changes are included in this PR?

This PR proposes to remove obsolete snprintf workaround for Python 3.9 on Windows.

### Are these changes tested?

I tested locally but do not have Windows environment. I will rely on CI.

### Are there any user-facing changes?

No.

* GitHub Issue: #48355

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

Author: HyukjinKwon

python/pyarrow/src/arrow/python/platform.h

@@ -32,10 +32,4 @@
 #  if _MSC_VER >= 1900
 #    undef timezone
 #  endif
-
-// https://bugs.python.org/issue36020
-// TODO(wjones127): Can remove once we drop support for CPython 3.9
-#  ifdef snprintf
-#    undef snprintf
-#  endif
 #endif