[simple theme] remove '-moz-border-radius' property

Support for the prefixed version (-moz-border-radius) was removed in Gecko
13.0 (Firefox 13.0 / Thunderbird 13.0 / SeaMonkey 2.10) [1].

[1] https://developer.mozilla.org/pt-BR/docs/Web/CSS/border-radius



Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>