Disable postcss-calc's noisy warnWhenCannotResolve option

While it sounds like a useful warning at first, it turns out the warnings it
prints are ones we're unlike to ever act on, such as adding percentages and
pixels, which seem fine to have. This resets to default behaviour, which leaves
the warning off.

webpack.config.js

@@ -243,7 +243,7 @@ module.exports = (env, argv) => {
                                     require("postcss-easings")(),
                                     require("postcss-strip-inline-comments")(),
                                     require("postcss-hexrgba")(),
-                                    require("postcss-calc")({warnWhenCannotResolve: true}),
+                                    require("postcss-calc")(),
 
                                     // It's important that this plugin is last otherwise we end
                                     // up with broken CSS.