auxdisplay: panel: Remove unused callback binding code

panel_bind_callback() has been compiled out with #if 0 since
the commit 630231776da4916e ("Staging: panel: remove support
for smartcards") in v2.6.29 and now has no callers. Remove
the dead function to reduce maintenance burden.

No functional change.

Signed-off-by: Miles Krause <mileskrause5200@gmail.com>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Willy Tarreau <willy@haproxy.com>
Link: https://patch.msgid.link/20260712-panel-remove-dead-callback-v1-1-1091ce75b8d5@gmail.com
[andy: update commit message as suggested by Geert]
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>