docs: Remove now-obsolete trusty64 box instructions.
docs: Remove now-obsolete trusty64 box instructions.

This annoying Vagrant bug hasn't been super relevant for over a year.

hackerkid/zulip,andersk/zulip,zulip/zulip,kou/zulip,rishig/zulip,dhcrzf/zulip,hackerkid/zulip,showell/zulip,hackerkid/zulip,zulip/zulip,tommyip/zulip,zulip/zulip,punchagan/zulip,showell/zulip,andersk/zulip,brainwane/zulip,dhcrzf/zulip,jackrzhang/zulip,rht/zulip,shubhamdhama/zulip,tommyip/zulip,brainwane/zulip,dhcrzf/zulip,rishig/zulip,timabbott/zulip,showell/zulip,rishig/zulip,brainwane/zulip,hackerkid/zulip,timabbott/zulip,andersk/zulip,kou/zulip,synicalsyntax/zulip,tommyip/zulip,brainwane/zulip,eeshangarg/zulip,punchagan/zulip,eeshangarg/zulip,hackerkid/zulip,andersk/zulip,rishig/zulip,eeshangarg/zulip,showell/zulip,tommyip/zulip,rht/zulip,hackerkid/zulip,dhcrzf/zulip,kou/zulip,rht/zulip,eeshangarg/zulip,punchagan/zulip,synicalsyntax/zulip,shubhamdhama/zulip,dhcrzf/zulip,dhcrzf/zulip,jackrzhang/zulip,zulip/zulip,brainwane/zulip,synicalsyntax/zulip,punchagan/zulip,kou/zulip,tommyip/zulip,brainwane/zulip,rht/zulip,shubhamdhama/zulip,rishig/zulip,punchagan/zulip,tommyip/zulip,kou/zulip,eeshangarg/zulip,punchagan/zulip,showell/zulip,zulip/zulip,showell/zulip,hackerkid/zulip,shubhamdhama/zulip,jackrzhang/zulip,timabbott/zulip,jackrzhang/zulip,synicalsyntax/zulip,timabbott/zulip,andersk/zulip,rht/zulip,rishig/zulip,andersk/zulip,zulip/zulip,synicalsyntax/zulip,synicalsyntax/zulip,timabbott/zulip,tommyip/zulip,rishig/zulip,andersk/zulip,brainwane/zulip,shubhamdhama/zulip,rht/zulip,jackrzhang/zulip,shubhamdhama/zulip,eeshangarg/zulip,kou/zulip,jackrzhang/zulip,synicalsyntax/zulip,dhcrzf/zulip,jackrzhang/zulip,shubhamdhama/zulip,showell/zulip,kou/zulip,zulip/zulip,timabbott/zulip,punchagan/zulip,eeshangarg/zulip,timabbott/zulip,rht/zulip