Remove references to Trusty-specific steps for Install from Source for Gazebo >= 9

[traversaro]

The tutorial to install Gazebo from source for Gazebo >= 9 in https://github.com/osrf/gazebo_tutorials/blob/master/install_from_source/tutorial_default.md#dart-support reports some Trusty-specific steps (adding ``ppa:libccd-debsandppa:fcl-debs`) that however do not make sense as Gazebo >= 9 does not support Trusty.

This steps may be confusing for users that try to install this PPA in non-Trusty Ubuntu distributions, and get some missing dependencies errors.

This is probably a "good first issue".

cc @HuitongIIT