Fix for circular imports in compile and runtime

Bug No: 1339814

get_runtime() is a redundant function not being currently used by any other
module. This is the only place in compile.py where runtime module is used.
Someone who needs to use Runtime() instance can directly import from
runtime.Runtime instead of using compile.get_runtime().

The runtime.py module however needs compile.py in several occassions.
So the import in runtime.py must be retained.

Change-Id: I6389e7210c10724bfc85dad7dad4693f733357f7