[Sets] Remove TwigSetProvider

[TomasVotruba]

Alternative to #1014: instead of ignoring the deprecated set objects in TwigSetProvider, drop the provider itself. It was the last SetProviderInterface implementation left here, after #1010 and #1012 removed the Symfony ones.

The provider only carried the Twig composer-based.php set, whose rules already declare their own twig/twig constraint - so nothing is lost by removing the ComposerTriggeredSet wrapper:

per-version set	covered in config/sets/twig/composer-based.php
twig112.php	RenameClassRector, twig/twig >=1.12 <1.40
twig127.php	RenameMethodRector, twig/twig >=1.27
twig134.php	SimpleFunctionAndFilterRector, bound to twig/twig 1.34
twig140.php, twig24.php, twig-underscore-to-namespace.php	RenameClassRector, twig/twig >=1.40
twig20.php	same >=1.40 rename, as both underscore names lead to the same namespaced class
twig30.php	AddReturnTypeDeclarationRector, twig/twig >=3.0

The 184 renames of twig-underscore-to-namespace.php were diffed entry by entry against the >=1.40 block; the only one not carried over is 'Twig_Tests_EnvironmentTest_Extension' => 'Twig\Tests\EnvironmentTest\Extensio', a class from Twig's own test suite with a truncated target name.

The per-version sets and the TwigSetList constants stay as they are, so anyone registering them by hand is unaffected.