[YSQL] Dropping table doesn't drop table index

[frozenspider]

postgres=# create table test(a int);
CREATE TABLE
postgres=# create unique index test_a on test(a);
CREATE INDEX
postgres=# drop table test;
DROP TABLE
postgres=# create table test(a int);
CREATE TABLE
postgres=# create unique index test_a on test(a);
ERROR:  relation "test_a" already exists

Moreover, we aren’t supporting DROP INDEX statement - we can't get rid of leftover indices:

postgres=# drop index test_a;
ERROR:  DROP INDEX not supported yet
LINE 1: drop index test_a;
             ^

[nocaway]

Need to add support for DROP INDEX.
According to Postgres's specification on drop table, indexes on table must ALWAYS be dropped.

DROP TABLE always removes any indexes, rules, triggers, and constraints that exist for the target table. However, to drop a table that is referenced by a view or a foreign-key constraint of another table, CASCADE must be specified. (CASCADE will remove a dependent view entirely, but in the foreign-key case it will only remove the foreign-key constraint, not the other table entirely.)