use builtin scrypt from node >= 12.8

[sidhujag]

https://github.com/web3-js/scrypt-shim/blob/master/src/index.js#L38
checks for fallback which I believe the maxmem was fixed to improve addressing from 32 to 53 bits. Can we enable this as of node 12.8 to remove scrypt as a dependency totally? Currently it still tries to find scrypt package if using node >= 12, but as of 12.8 we can depend on the built-in one. This ensures that no scrypt package is required.

See nodejs/node#28755
and release:
https://nodejs.org/ru/blog/release/v12.8.0/

[michaelsbradleyjr]

See: web3/web3.js#3284 (comment).