narrow: Remove "subscribe" button for guests for emptry streams.
narrow: Remove "subscribe" button for guests for emptry streams.

This button didn't work, because the backend blocks subscribing, so it
was just confusing.

Fixes an issue reported in #11743.

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