remove nativeint

[bobzhang]

According to the docs:

This module provides operations on the type nativeint of signed 32-bit integers (on 32-bit platforms) or signed 64-bit integers (on 64-bit platforms). This integer type has exactly the same width as that of a pointer type in the C compiler. All arithmetic operations over nativeint are taken modulo 232 or 264 depending on the word size of the architecture.

Nativeint does not make sense on JS backend