util: cleanup internalUtil.deprecate

There were two functions `deprecate` and `_deprecate`
that were really just aliases of each other. Simplify

PR-URL: https://github.com/nodejs/node/pull/11450


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>