proxy: delete destroyed partitions from forwarded_partition_request

When a forwarded partition is requested in destroyed state, delete
the entry from forwarded_partition_request table (after getting reply
from remote server)
5 jobs for feat/destroy_frontend_instance (queued for 2 seconds)
Status Job ID Name Coverage
  External
running SlapOS-Master.UnitTest-Master

12881:40:06

passed SlapOS.Eggs.UnitTest-Jerome.Python2

00:13:10

passed SlapOS.Eggs.UnitTest-Jerome.Python3

00:14:23

passed SlapOS.Eggs.UnitTest-Master.Python2

00:13:13

passed SlapOS.Eggs.UnitTest-Master.Python3

00:14:35