Revert "add MaskExpr"

This reverts commit 0cbf4ac.

Signed-off-by: Andrew Duffy <[email protected]>

Author: a10y

vortex-expr/src/exprs/mask.rs

@@ -11,7 +11,6 @@ pub(crate) mod is_null;
 pub(crate) mod like;
 pub(crate) mod list_contains;
 pub(crate) mod literal;
-mod mask;
 pub(crate) mod merge;
 pub(crate) mod not;
 pub(crate) mod operators;