boards: x86: minnowboard: Do not assign promptless X86_NO_SPECTRE_V4

Assignments to promptless symbols have no effect. Flagged by
https://github.com/zephyrproject-rtos/zephyr/pull/20742.

From looking at
http://www.mouser.com
  /catalog/specsheets/CircuitCO_595-MINNOWMAX-DUAL%20Data%20Sheet.pdf
, this board uses an Intel Atom E3815 or an Intel Atom E3825 CPU.

Both of those CPUs are listed as vulnerable to all three variants of
Meltdown and Spectre in
https://www.techarp.com/guides/complete-meltdown-spectre-cpu-list/6/

.

All the X86_NO_SPECTRE_V* symbols default to n (and it seems nothing
turns them on at the moment), so just remove the assignment, which will
be a no-op.

Also remove the comment re. Spectre V4, as it seems this board is
vulnerable to more than that, though I could've missed something.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>