Revert "caller_cx only works for >= 5.6.0 (for now)"

This reverts commit 264857d3495cd5821fa72069db1f3971c869eb82.
Revert "RT-93521 Add modified patch from Ben Morrow to support caller_cx."

This reverts commit 84d202261d0b8e63d49c1e30cc22ca1d3e4bdf62.