Remove redundant font-family rule in button

Remove explicit font-family definition from the button scss. We don't need it because we get it as part of the included `govuk-font-regular` mixin which defines the font for us.

This will remove a redundant duplicated font-family rule and also mean that the button will correctly fall back to the print font when printed.

src/button/_button.scss

@@ -48,7 +48,6 @@
     color: $govuk-button-text-colour;
     background-color: $govuk-button-colour;
     box-shadow: 0 $button-shadow-size 0 $govuk-button-shadow-colour; // s0
-    font-family: $govuk-font-stack;
     text-align: center;
     vertical-align: top;
     cursor: pointer;