Remove custom gutter overwrite on rg-sized screens and bigger

[oliverschuerch]

This custom legacy rules (located under styles/src/layouts/portal/_grid.scss) do block every attempt to set the gutter width through the utility classes g-n, gx-n and gy-n, on screens bigger/equal to the rg breakpoint. This happens, because the rules override the bootstrap behaviour which uses the custom-property --bs-gutter-x, with a hardcoded/fixed value.