SQL server 2016 restore impact of mirroring configuration?
You cannot just delete or drop db when mirroring is configured. You have to remove mirroring alter database dbname set partner off.

Then drop database on primary.

Then restore from the desired backup.

Then reset up mirroring - take full and log backup and restore it to server B with NO_RECOVERY.

Remember : Mirroring is deprecated technology. So you should either use basic availability group or if you are on Enterprise edition then go with AlwaysON.