loader/efi: Linker sets not needed.

We don't need linker sets listed as sections. They are explicitly
included in the objcopy we use to create the .efi file. This practice
was added in 2002 by peter@ in a6d81d83a280 to make ia64 builds
self-hosted. However, it was added back to the objcopy in 2010 by rpaulo
in 8df7a05edd36 for i386 EFI support, though the ldscript file then
retained them needlessly. The gcc/binutils bug having been fixed in the
interim. We've not needed them since then, but the redundancy didn't
matter.

Sponsored by: Netflix
Reviewed by: tsoome, kevans
Differential Revision: https://reviews.freebsd.org/D44061