Remove build script

[Anders429]

The only thing the build script does is check whether 128-bit integers are supported. However, serde stopped supporting conditional compilation based on 128-bit integers with serde-rs/serde#2600, released in version 1.0.187. The macro used here technically still exists, but it is a no-op.

Since this crate doesn't support rustc versions older than 1.40 (and never has), it seems reasonable to remove this from the build script.