[PowerPC] Remove obsolete MK_LOADER_FORCE_LE

In D12421, the ability to compile stand/ in little-endian was added, with the
intention to extend loader.kboot to run in Petitboot.

However, no further work was done, as the kernel then gained self-execution
capabilities as Petitboot was taught to load FreeBSD kernels directly.

The FreeBSD installer on powerpc64 (on POWER8 and POWER9) uses
/boot/etc/kboot.conf instead of loader.

As this option does nothing but cause stand/ to be miscompiled and actively
causes confusion, remove it.

(I have a functioning petitboot loader in my local tree, however, it turned
out to be quite inconvient to use due to the current petitboot plugin design
so I put it on hold.)

Reviewed by: emaste, imp, jhibbits
Sponsored by: Tag1 Consulting, Inc.
Differential Revision: https://reviews.freebsd.org/D26430