Remove unused feature_flags.collapsible field.
Remove unused feature_flags.collapsible field.

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