psread.c: Remove (originally) unreached code that broke autotrace

Commit b2a6cbb563d049a033508ec85fef583d14f38ac6 introduced the code
with check
   if (sp < sizeof(stack)/sizeof(stack[0]))

But as `stack` is a pointer, the result was comparing `sp < 0`, which
can never occur (sp >= 0). Hence this code block was never executed.

Commit a2c8bab1914abe2d6d1c012d8b57bf6444ce9e02 changed the code to be:
    if (sp < stacktop)

Which I can only guess is what it should be, *if* the code in the
code block worked with the current code.
Merge pull request #1698 from jtanx/autotrace

psread.c: Remove (originally) unreached code that broke autotrace