debug: revert export of trace.

Both because the argument check wrapper is not properly tail-call
eliminated by Lua 5.1, and because as a core function callback
the function signature is fixed already, there's no good reason
to check arguments on debug.trace.
* specs/debug_spec.yaml (trace): Remove argument check behaviour
specifications.
* lib/std/debug.lua (trace): Remove the export wrapper.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>