Drop physical Data Guard instance

SQL> select open_mode from v$database;

OPEN_MODE
--------------------
MOUNTED

SQL> alter database recover managed standby database cancel;

Database altered.

SQL> !hostname
cstsdr-01

SQL> shutdown immediate
ORA-01109: database not open


Database dismounted.
ORACLE instance shut down.
SQL> startup restrict mount
ORACLE instance started.

Total System Global Area 1.5032E+10 bytes
Fixed Size                  4504368 bytes
Variable Size            6073353424 bytes
Database Buffers         8824815616 bytes
Redo Buffers              129712128 bytes
Database mounted.
SQL> !hostname
cstsdr-01

SQL> drop database;