[Refactoring] Remove unused variable from Rooms

[rkowase]

Issue

• n/a

Overview (Required)

In this PR, I removed unused variable from Rooms to fix a lint warning. I'm not sure anyone plan to use this unused variable in the future, but I think it should be removed for now as _YAGNI principal_.

Links

• Related PR: https://github.com/DroidKaigi/conference-app-2022/pull/879

Screenshot