remove NaN from BigInt

Author: mununki

jscomp/others/js_bigint.res

@@ -1,11 +1,5 @@
 /*** JavaScript BigInt API */
 
-@val
-/**
-The special value "Not a Number". See [`NaN`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/NaN) on MDN.
-*/
-external _NaN: bigint = "NaN"
-
 @val
 /**
 Parses the given `string` into a `bigint` using JavaScript semantics. Return the