tools/ci: Fix KeyError when running tools/ci/generate-dockerfiles.
tools/ci: Fix KeyError when running tools/ci/generate-dockerfiles.

This KeyError occurs because we remove extra_packages from
images.yml because it was no longer needed in a486872a8ed28a3a56abf5498a75a9b943a82ebd.

showell/zulip,zulip/zulip,kou/zulip,zulip/zulip,rht/zulip,andersk/zulip,brainwane/zulip,showell/zulip,rht/zulip,showell/zulip,punchagan/zulip,zulip/zulip,rht/zulip,eeshangarg/zulip,hackerkid/zulip,hackerkid/zulip,punchagan/zulip,brainwane/zulip,brainwane/zulip,kou/zulip,showell/zulip,brainwane/zulip,zulip/zulip,andersk/zulip,punchagan/zulip,hackerkid/zulip,kou/zulip,showell/zulip,kou/zulip,eeshangarg/zulip,rht/zulip,zulip/zulip,eeshangarg/zulip,kou/zulip,brainwane/zulip,showell/zulip,punchagan/zulip,rht/zulip,eeshangarg/zulip,zulip/zulip,rht/zulip,kou/zulip,kou/zulip,eeshangarg/zulip,eeshangarg/zulip,eeshangarg/zulip,hackerkid/zulip,showell/zulip,rht/zulip,punchagan/zulip,andersk/zulip,hackerkid/zulip,hackerkid/zulip,andersk/zulip,punchagan/zulip,brainwane/zulip,andersk/zulip,zulip/zulip,brainwane/zulip,andersk/zulip,punchagan/zulip,hackerkid/zulip,andersk/zulip