Revert "Add API to set connection interval at GATT connection"

Revert submission 2352067-bt_gatt_conn_interval_api

Reason for revert: This change cannot be supported natively in the stack. See b/267229740#comment3 for more context.

Bug: 267229740
Test: N/A - API revert

Reverted changes: /q/submissionid:2352067-bt_gatt_conn_interval_api

Change-Id: I94a29b8b60866ddcb3c893f5493f6e571eb58cfe