minor: Fix comment in send_message_backend.
minor: Fix comment in send_message_backend.

The `send_message_backend` function no longer
calls `recipient_for_emails`.

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