TASK: Tweak Symfony polyfill replacements
TASK: Tweak Symfony polyfill replacements

The replacements added according to the symfony/polyfill README (and
tweaked recently to actually appear in the split manifest) lead to an
installation issue (https://github.com/composer/composer/issues/9834)

Thus this removes the PHP 5 polyfills (those not being replaced should
not be an issue) to fix that. Also, since we require PHP 7.1, the 7.2
polyfill must not be replaced.

neos/flow,neos/flow,neos/flow,neos/flow