Disable cython profiling by default

Enabling cython profiling adds some overhead.

http://cython.readthedocs.io/en/latest/src/tutorial/profiling_tutorial.html

Imported from Jira [BEAM-4070](https://issues.apache.org/jira/browse/BEAM-4070). Original Jira may contain additional context.
Reported by: boyuanz.

Start by locating the Cython profiling configuration in the Apache Beam Python code and review the linked Cython profiling documentation. Done means profiling is disabled by default while an explicit opt-in remains available, with the relevant tests updated or added if the repository has coverage for this setting.

Written by the indexing model from the issue text.