package/python-pymodbus: drop asyncio files for python 2.x to fix pycompile issue

Pymodbus has optional support for asyncio.  Pycompile unfortunately errors
out on these files when running under Python 2.x:

../scripts/pycompile.py ..
error:   File "/usr/lib/python2.7/site-packages/pymodbus/client/asynchronous/asyncio/__init__.py", line 257
    yield from self._connect()
             ^
SyntaxError: invalid syntax

As a workaround, simply drop the unusable files from TARGET_DIR if building
for python 2.x.

Fixes:
http://autobuild.buildroot.net/results/cc4/cc48927cbe9ae6c2d8b12d65467ec40df82febf6/



Signed-off-by: Peter Korsgaard <peter@korsgaard.com>