node tests: Remove unecessary with_field call in pm_list.
node tests: Remove unecessary with_field call in pm_list.

After eeee6edf41222a7c288b752c787e8630f479a7f3, `is_all_privates`
isn't called from the `update_private_messages`
path, and doesn't need to be stubbed.

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