remove extension check in FixerInterface::supports

Extension check should be done in Finder.
If one want to fix only php file he should set it in Finder.
Now we have a Finder that finds .php, .twig etc - but almost all fixers will fix only .php.
What's more - if one have his own extension to fix, like .php5, .inc or .module and set it in Finder, then fixers still do not fix them.