Removed the option to switch to a different templating engine

This option was introduced in the early days of the Symfony development
when Twig was not yet the recommended engine, to let users remove the
dependency on Twig at the expense of rewriting all templates for their
engine.
Making the code ugly and inconsistent with the Symfony ecosystem to
implement a feature which is barely usable is not worth it.
minor #1613 Removed the option to switch to a different templating engine (stof)

This PR was merged into the 2.0.x-dev branch.

Discussion
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Removed the option to switch to a different templating engine

This option was introduced in the early days of the Symfony development when Twig was not yet the recommended engine, to let users remove the dependency on Twig at the expense of rewriting all templates for their engine.
Making the code ugly and inconsistent with the Symfony ecosystem to implement a feature which is barely usable is not worth it.

Commits
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f9abc64 Removed the option to switch to a different templating engine