lttng-tools: Revert wrong enforcement of Python 3.0 use

lttng-tools: Revert wrong enforcement of Python 3.0 use

The code has support and checks for Python 2 and Python 3 however
during the development of 2.7 release a requirement for Python 3 was
included.

The Python 3 is required for runtime tests to run however those are
not in use now so we can just disable the enforcement for now.

This reverts the commit restoring the Python 2 support.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Ross Burton <ross.burton@intel.com>