Clean out duplicate ISSUE template

.github/ISSUE_TEMPLATE/Feature_Request.md

-<!--
-
-Before you request a feature, please realize this project is a wrapper for
-LibSass so we can't add any language features here. If you want to add a feature
-to the Sass language, please open an issue on https://github.com/sass/sass/issues/new
-
-If you want a CLI feature, please make sure that it is supported by https://github.com/sass/sass
-
-Thanks for making node-sass better!
-
--->