Hello,
there is 2 options:
- change the header style from wp-admin >> appearance >> customize >> header >> header style.
select sleek header style. only default and generic has the top menu option.
- goto wp-admin >> appearance >> customize >> custom css.
paste this code if you want to hide only sign up button.
ul.topmenu li:last-child {display: none !important;}
refer:
http://prntscr.com/ps2pov
if you want to hide the whole top menu list icon then use this one
ul.topmenu {display: none !important;}
refer:
http://prntscr.com/ps2q2d