uasyncio.core: logging is not longer hard dependency, remove it.

If user is going to enable logging in particular app, they should depend
on it themselves.

uasyncio.core/metadata.txt

@@ -3,4 +3,3 @@ type = package
 version = 1.5
 author = Paul Sokolovsky
 long_desc = Lightweight implementation of asyncio-like library built around native Python coroutines. (Core event loop).
-depends = logging

uasyncio.core/setup.py

@@ -17,5 +17,4 @@ setup(name='micropython-uasyncio.core',
       maintainer_email='micro-python@googlegroups.com',
       license='MIT',
       cmdclass={'optimize_upip': optimize_upip.OptimizeUpip},
-      packages=['uasyncio'],
-      install_requires=['micropython-logging'])
+      packages=['uasyncio'])