flatpickr: Hide bottom arrow in dark mode.
flatpickr: Hide bottom arrow in dark mode.

Previous change didn't hide it in dark mode since this css was
being overridden.

andersk/zulip,kou/zulip,zulip/zulip,zulip/zulip,rht/zulip,andersk/zulip,kou/zulip,eeshangarg/zulip,andersk/zulip,rht/zulip,zulip/zulip,eeshangarg/zulip,rht/zulip,eeshangarg/zulip,rht/zulip,kou/zulip,zulip/zulip,eeshangarg/zulip,zulip/zulip,zulip/zulip,eeshangarg/zulip,kou/zulip,kou/zulip,rht/zulip,andersk/zulip,kou/zulip,rht/zulip,zulip/zulip,kou/zulip,andersk/zulip,rht/zulip,eeshangarg/zulip,andersk/zulip,eeshangarg/zulip,andersk/zulip