KVM: x86: Turn off KVM_WERROR by default for all configs

Don't enable KVM_WERROR by default for x86-64 builds as KVM's one-off
-Werror enabling is *mostly* superseded by the kernel-wide WERROR, and
enabling KVM_WERROR by default can cause problems for developers working
on other subsystems.  E.g. subsystems that have a "zero W=1 regressions"
rule can inadvertently build KVM with -Werror and W=1, and end up with
build failures that are completely uninteresting to the developer (W=1 is
prone to false positives, especially on older compilers).

Keep KVM_WERROR as there are combinations where enabling WERROR isn't
feasible, e.g. the default FRAME_WARN=1024 on i386 builds generates a
non-zero number of warnings and thus errors, and there are far too many
warnings throughout the kernel to enable WERROR with W=1 (building KVM
with -Werror is desirable (with a sane compiler) as W=1 does generate
useful warnings).

Opportunistically drop the dependency on !COMPILE_TEST as it's completely
meaningless (it was copied from i195's -Werror Kconfig), as the kernel's
WERROR is explicitly *enabled* for COMPILE_TEST=y kernel's, i.e. enabling
-Werror is obviosly not dependent on COMPILE_TEST=n.

Reported-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/all/20231006205415.3501535-1-kuba@kernel.org
Link: https://lore.kernel.org/r/20231018151906.1841689-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>