objtool: Remove --no-unreachable for noinstr-only vmlinux.o runs

For (!X86_KERNEL_IBT && !LTO_CLANG && NOINSTR_VALIDATION), objtool runs
on both translation units and vmlinux.o.  The vmlinux.o run only does
noinstr/noret validation.  In that case --no-unreachable has no effect.
Remove it.

Note that for ((X86_KERNEL_IBT || LTO_CLANG) && KCOV), --no-unreachable
still gets set in objtool-args-y by scripts/Makefile.lib.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/05414246a0124db2f21b0d071b652aa9043d039d.1742852847.git.jpoimboe@kernel.org