Remove migration of linuxbrew-core formulae

Follow up to Homebrew/brew#21190. We enabled migration over 4 years ago
since Homebrew/brew#12248, which was part of brew 3.3.0. So at least 2
major brew releases have passed and all active Linuxbrew users would
have been migrated already.

There are only 25 formulae left so anyone who hasn't migrated can just
reinstall formulae if impacted.