delete-trailing-whitespace misbehaves in scheme-mode

[Jose A. Ortega]

In a buffer with scheme-mode active, delete-trailing-whitespace treats
a traling vertical bar character (|) as trailing whitespace (that is,
the character is deleted when invoking delete-trailing-whitespace,
either interactively or as a write hook). Other modes (elisp, lisp, c,
fundamental) seem to behave correctly.

This behaviour is also present in Emacs 21.

(tested using emacs -q, both in emacs 22 and 21)