[buteo-sync-plugins-social] Remove code for supporting multiple accou…

…ntIds. Contributes to JB#51507

These changes don't affect functionality, the aim is only to improve
code clarity.

Various parts of the code (notably in the finalCleanup() method) were
already assuming that the adapter would service only a single account
during its lifetime.

This change removes the additional code needed to support multiple
accountIds, which was going unused.