vmlinux.lds.h: Remove entry to place init_task onto init_stack

Since commit 0eb5085c ("arch: remove ARCH_TASK_STRUCT_ON_STACK")
there is no option that would allow placing task_struct on the stack.
Remove the unused linker script entry.

Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20241217202745.1402932-2-benjamin@sipsolutions.net