css: Remove unused portico css.
css: Remove unused portico css.

This *-page-header CSS all dates from a nearly-original version of the
portico design system, and hasn't been functional in years.

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