objtool: Remove --unret dependency on --rethunk

With unret validation enabled and IBT/LTO disabled, objtool runs on TUs
with --rethunk and on vmlinux.o with --unret.  So this dependency isn't
valid as they don't always run on the same object.

This error never triggered before because --unret is always coupled with
--noinstr, so the first conditional in opts_valid() returns early due to
opts.noinstr being true.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/c6f5635784a28ed4b10ac4307b1858e015e6eff0.1741975349.git.jpoimboe@kernel.org