Remove mymonero from chosing a Monero client. Add GUI

Created by: Jaqueeee

MyMonero as been in a not working state for >3 weeks and shouldn't be promoted here. https://www.reddit.com/r/Monero/comments/5rzcjf/a_couple_of_suggested_tweaks_to_make_monero_more/?st=iyr3ufj8&sh=c9dae848

Added link to Beta GUI