python/setup.py: remove unneeded wheel dependency

wheel is required since version 3.13.0 and
https://github.com/protocolbuffers/protobuf/commit/ff92cee10bcae7533b573368f448e782fbd43f39

This will result in the following build failure when cross-compiling:

Download error on https://pypi.org/simple/wheel/: unknown url type: https -- Some packages may not be found!
Couldn't find index page for 'wheel' (maybe misspelled?)
Download error on https://pypi.org/simple/: unknown url type: https -- Some packages may not be found!
No local packages or working download links found for wheel

Remove wheel requirement from setup.py as it is only needed by
release.sh, not by setup.py

Fixes:
 - http://autobuild.buildroot.org/results/371c686a10d6870933011b46d36b1879d29046b9

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>

python/setup.py

@@ -293,7 +293,6 @@ if __name__ == '__main__':
           'build_py': build_py,
           'test_conformance': test_conformance,
       },
-      setup_requires = ['wheel'],
       install_requires=install_requires,
       ext_modules=ext_module_list,
   )