remove superfluous interface
nit's
revert arg-types + return-type checking (not needed)
Merge branch '7813-udf-keyspace' into 8053-udaf-v2

Conflicts:
	src/java/org/apache/cassandra/cql3/statements/CreateFunctionStatement.java
	src/java/org/apache/cassandra/cql3/statements/DropFunctionStatement.java