Replace ReferentialAction enum in DROP statements

[stepancheg]

There's <drop behavior> definition in the grammar, so corresponding enum seems appropriate:

pub enum DropBehavior {
    Restrict,
    Cascade,
}

vs

pub enum ReferentialAction {
    Restrict,
    Cascade,
    SetNull,
    NoAction,
    SetDefault,
}

Context: ALTER TABLE DROP COLUMN sqlparser does not support RESTRICT. I wanted to add it, but adding ReferentialAction there too might be misleading, so here I introduce an enum.

[iffyio]

Thanks @stepancheg! This improvement sounds reasonable to me!
cc @alamb