Refactor: Use backend-endpoint function instead of helper function.
Refactor: Use backend-endpoint function instead of helper function.

Use backend-endpoint function instead of helper function in
`test_realm_linkifiers.py` so that tests are more end-to-end.

The removed helper function: `do_add_linkifier` is tested in
`zerver/tests/test_events.py`.

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