[1.19] Remove `ENABLE_CUSTOM_CALLS`

[ti-mo]

As a follow-up to #38480, remove ENABLE_CUSTOM_CALLS and all of its references and code.

[joestringer]

@ti-mo are you using milestones to track this? In order to properly set this as a release blocker, we need to set the milestone to the target version. Perhaps it would be better to create a project to track clang-free tasks rather than using the milestone?