revert: flatten untyped recursive NULL casts in bq compiler

Author: shuoweil

third_party/bigframes_vendored/ibis/backends/sql/compilers/bigquery/__init__.py

@@ -540,15 +540,6 @@ def visit_TimestampFromUNIX(self, op, *, arg, unit):
 
     def visit_Cast(self, op, *, arg, to):
         from_ = op.arg.dtype
-        if to.is_null():
-            return sge.Null()
-        if arg is NULL or (
-            isinstance(arg, sge.Cast)
-            and getattr(arg, "to", None) is not None
-            and str(arg.to).upper() == "NULL"
-        ):
-            if to.is_struct() or to.is_array():
-                return sge.Cast(this=NULL, to=self.type_mapper.from_ibis(to))
         if from_.is_timestamp() and to.is_integer():
             return self.f.unix_micros(arg)
         elif from_.is_integer() and to.is_timestamp():