fix(mvebu-6.18): drop SysRq-via-BREAK patch, fixed upstream in 6.18.35

The patch general-serial-8250-fix-sysrq-break-dw-apb.patch was added
in PR #9750 to restore SysRq-via-BREAK on dw-apb-uart (Armada 38x /
helios4) after the same regression was backported from Linux 7.0 into
the 6.18.y stable series.

linux-6.18.y has since received the upstream fix:

  "serial: 8250_dw: dispatch SysRq character in dw8250_handle_irq()"

— the same fix that was applied in 7.0.12 and removed the analogous
rockchip64-7.0 copy of this patch in #9956.

With linux-6.18.35 both hunks now reject:
  drivers/tty/serial/8250/8250_dw.c   Hunk #1 FAILED
  drivers/tty/serial/8250/8250_port.c Hunk #1 FAILED

Verified on helios4 (Armada 388, dw-apb-uart): kernel build fails at
patching stage (11/12 applied, 1 failed_apply) without this removal.

Assisted-by: Claude:claude-sonnet-4.6

Author: iav