debug: fix argerror diagnostics on luajit.

Luajit seems to treat `return error` (where error never returns
anyway) as subject to some kind of optimisation that loses a call
stack level by the time `error` looks up the line number for the
targetted level. Removing the `return` results in the same call
stack frame reference in luajit, 5.2 and 5.1.
* lib/std/debug.lua (argerror): Drop the extraneous `return`
statement.

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