Improves the "your reservations" look in seantis.reservation. A lot.

Removing the `font-size` variable causes that this font will not adapt to the defined font-size in the site configuration.
See: https://github.com/OneGov/plonetheme.onegov/blob/master/plonetheme/onegov/resources/sass/post_variables.scss
I would try to use `$font-size-18`(=>`<h2>`) or `$font-size-36`(=>`<h1>`).
(Or if you need a size between this, you could use something like `($font-size-18*1.5)`)
But I think for the appearance of the website it would look better to use the same font-sizes everywhere...