If you do this:
#masthead {
display: none;
}
…then the primary navigation menu will not be there on mobile devices, because the mobile primary navigation is inside of #masthead.
I did it like this (to remove it from start-page, on all pages do it without .home):
.home #site-branding{display: none;}
.home #masthead {
background-color: #404040;
height: 0px;
padding: -1px;
margin: -100px;
}
Then i had to reposition the mobile navigation:
.home #header-left-menu{
background-color: #d0d0d0;
/*position: relative;*/
/*right: -5vw;*/
padding: 4px;
margin: 10px;
}
For some mobile devices i had to do the same within:
@media screen and (-webkit-min-device-pixel-ratio:0) {...}