Remove Rubocop 0.55 shim

Summary:

Once facebook/between-meals#127 and
facebook/grocery-delivery#66 have been merged, this
version shim can go. We now take an allowlist-based approach with 1.3.1+, so
shouldn't need new shims going forward.

Differential Revision: D45874516

fbshipit-source-id: 15f5d7977dcdb5a426d38cc841a6d54c670b2133

Author: dafyddcrosby