loader/kboot: simplify linker set inclusion a little

Linker set sections are included by default. No need to do so
explicitly. These were bogusly copied from the efi ldscripts. They were
there due to a workaround introduced in 2002 by peter@ for a gcc
upgrade, but whatever bugs necessitated it were filed by 2010 when
rpaulo@ imported the i386 support (though they were copied even though
the objcopy retained them correctly, the gcc bug having been
fixed). They've never been needed.

Sponsored by: Netflix
Reviewed by: tsoome, kevans
Differential Revision: https://reviews.freebsd.org/D44060