streams: Extract removing_someone_else logic to a function.
streams: Extract removing_someone_else logic to a function.

This commit extracts removing_someone_else logic to the function
'check_if_removing_someone_else' for better readability.

This is a follow up to 2187c84.

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