bug#5546: xterm-background-mode-changed undefined/unused

[Dan Nicolaescu]

Glenn Morris <rgm@gnu.org> writes:

  > Severity: minor
  > 
  > Compiling lisp/term/xterm.el warns:
  > 
  > In xterm-maybe-set-dark-background-mode:
  > xterm.el:682:11:Warning: assignment to free variable
  >     `xterm-background-mode-changed'
  > 
  > This variable is never defined, but also seems never to be used.

Fixed

Thanks!