Don't print `Stack depth: ...` messages unless requested with newrelic.special
show_execute_stack_depth.

Change of strategy: don't print `Stack depth: ...` messages. Stack depth can
can be inferred from the level of indentation used in `nr_php_show_exec` and
`nr_php_show_exec_return` - both use `nr_php_show_exec_indentation`, which
uses `NRPRG(php_cur_stack_depth)` to calculate indentation level.