Eliminate have_scrolled_away_from_top boolean.
Eliminate have_scrolled_away_from_top boolean.

The `have_scrolled_away_from_top` logic goes way back
to November 2012.

Now we unconditionally load older messages when we scroll
to the top of the feed.  Before this bug, you could get
"stuck."  It was a bit difficult to reproduce, but with
the right combination of render window sizes and batch
sizes, you would hit the home key quickly and hit the top
of the feed in a way that the flag got in the way of
going back in history.

Fixes #6628

synicalsyntax/zulip,punchagan/zulip,eeshangarg/zulip,tommyip/zulip,shubhamdhama/zulip,synicalsyntax/zulip,Galexrt/zulip,kou/zulip,mahim97/zulip,zulip/zulip,Galexrt/zulip,brainwane/zulip,timabbott/zulip,brockwhittaker/zulip,tommyip/zulip,rishig/zulip,shubhamdhama/zulip,punchagan/zulip,eeshangarg/zulip,brockwhittaker/zulip,brainwane/zulip,timabbott/zulip,mahim97/zulip,brainwane/zulip,punchagan/zulip,shubhamdhama/zulip,timabbott/zulip,zulip/zulip,Galexrt/zulip,andersk/zulip,punchagan/zulip,Galexrt/zulip,dhcrzf/zulip,mahim97/zulip,brainwane/zulip,kou/zulip,rishig/zulip,punchagan/zulip,andersk/zulip,showell/zulip,andersk/zulip,timabbott/zulip,andersk/zulip,mahim97/zulip,jackrzhang/zulip,synicalsyntax/zulip,rishig/zulip,eeshangarg/zulip,jackrzhang/zulip,andersk/zulip,eeshangarg/zulip,dhcrzf/zulip,brockwhittaker/zulip,brockwhittaker/zulip,rht/zulip,synicalsyntax/zulip,shubhamdhama/zulip,eeshangarg/zulip,rishig/zulip,dhcrzf/zulip,showell/zulip,hackerkid/zulip,zulip/zulip,tommyip/zulip,Galexrt/zulip,showell/zulip,jackrzhang/zulip,showell/zulip,zulip/zulip,synicalsyntax/zulip,kou/zulip,dhcrzf/zulip,timabbott/zulip,showell/zulip,synicalsyntax/zulip,punchagan/zulip,jackrzhang/zulip,rishig/zulip,zulip/zulip,Galexrt/zulip,brainwane/zulip,mahim97/zulip,synicalsyntax/zulip,andersk/zulip,kou/zulip,mahim97/zulip,rht/zulip,hackerkid/zulip,kou/zulip,timabbott/zulip,andersk/zulip,shubhamdhama/zulip,tommyip/zulip,dhcrzf/zulip,eeshangarg/zulip,hackerkid/zulip,shubhamdhama/zulip,hackerkid/zulip,rishig/zulip,eeshangarg/zulip,tommyip/zulip,showell/zulip,zulip/zulip,timabbott/zulip,rishig/zulip,jackrzhang/zulip,showell/zulip,hackerkid/zulip,brainwane/zulip,brainwane/zulip,rht/zulip,Galexrt/zulip,brockwhittaker/zulip,jackrzhang/zulip,shubhamdhama/zulip,rht/zulip,rht/zulip,punchagan/zulip,tommyip/zulip,dhcrzf/zulip,kou/zulip,rht/zulip,rht/zulip,brockwhittaker/zulip,kou/zulip,zulip/zulip,dhcrzf/zulip,jackrzhang/zulip,hackerkid/zulip,hackerkid/zulip,tommyip/zulip