Use API_VERSION instead of LANGUAGE_VERSION in IEEE754 tests

See `ExpressionCodegen.genEqualsForExpressionsPreferIeee754Arithmetic`:
the behavior here actually depends on the API version, not any language
feature

compiler/testData/codegen/box/ieee754/nullableDoubleEqualsLV13.kt

@@ -1,5 +1,3 @@
-// LANGUAGE_VERSION: 1.3
-
 fun myEquals(a: Double?, b: Double?) = a == b
 
 fun myEquals1(a: Double?, b: Double) = a == b
@@ -24,4 +22,4 @@ fun box(): String {
     if (!myEquals0(0.0, 0.0)) return "fail 9"
 
     return "OK"
-}
+}