cql3: expr: Remove repetition from expr::get_elements
cql3: expr: Remove repetition from expr::get_elements

There was some repeating code in expr::get_elements family
of functions. It has been reduced into one function.

Signed-off-by: Jan Ciolek <b66a6f3989e26cb552db14b7ab163c3a50292e24@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla