[Python] Remove gcc 4.9 compatibility code

Since we now require a C++17-compliant compiler, we don't support gcc 4.9 anymore. The following code can probably be simplified:
https://github.com/apache/arrow/blob/619b034bd3e14937fa5d12f8e86fa83e7444b886/python/pyarrow/src/arrow/python/datetime.cc#L41