chore(arel): remove outdated variable Arel::Visitors::VISITORS

[shooma]

reason: https://github.com/rails/arel/commit/62e3596db89570b9d48b13c9966d754d5859d0dc#diff-e53edfe0dc990ee54addfcc9232219ee

For example, application based on rails 5.2.2 can't establish connection through hana adapter because variable VISITORS is absent

As I understand, this variable is unused in Arel for last several years