remove CONSTRUCTORS from MIPS uboot linker script

The linker script CONSTRUCTORS keyword is only meaningful "when linking
object file formats which do not support arbitrary sections, such as
ECOFF and XCOFF"[1] and is ignored for other object file formats.

LLVM's lld does not yet accept (and ignore) CONSTRUCTORS, so just remove
CONSTRUCTORS from the linker script as it has no effect.

[1] https://sourceware.org/binutils/docs/ld/Output-Section-Keywords.html