portico: Remove unused Stamp font-face.
portico: Remove unused Stamp font-face.

This was used for the very early-stage Zulip "ask for invite" form,
which was built around a stamped envelope concept.  The form was
removed from Zulip a couple years ago in
bded0d9d541e44662f15e632d6a68ea7b2a76781, but this CSS was missed in
the removal.

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