powerpc: Remove flag CTLFLAG_TUN from sysctl knob hw.platform

Prior to change [1] this flag is useless but harmless. After the change
plat_name[] will be fetched from kernel environment after invoking the
platform probe function platform_probe_and_attach(). The probe function
runs at early boot stage prior to mi_startup() thus it is too late and
pointless to set plat_name[] after the probe.

Nathan mentioned that the logic to specify the platform pre-dates the
powerpc64 work, and is from the original pre-FDT Book-E bringup from
like 2008, so it's irrelevant these days. Instead of fixing setting the
sysctl knob hw.platform, let's clean it up now.

[1] 3da1cf1e88f8 Extend the meaning of the CTLFLAG_TUN flag to ...

Discussed with: nwhitehorn
Reviewed by: olce (previous version), jhibbits, PowerPC
MFC after: 5 days
Differential Revision: https://reviews.freebsd.org/D48897

(cherry picked from commit b61fbbed73ea3bf0c84589b56cca160c46a3739d)
(cherry picked from commit 7775f4c5c75221cd9b2aadf12fb1d9a329c08be4)