Remove unnecessary dependency on Material

[nikodembernat]

Currently, when adding this package to a project that doesn't use the Material Design there is a warning:

package:simple_icons has uses-material-design: true set but the primary pubspec contains uses-material-design: false. If the application needs material icons, then uses-material-design must be set to true.

It has no other effects besides being annoying, but I would appreciate no unnecessary issues in my debug console.

Edit: Also, there is no reason to import the whole package:flutter/material.dart in icon_data.dart - package:flutter/widgets.dart is sufficient.