KVM: x86: Drop ASSERT() on I/O APIC EOIs being only for LEVEL_to WARN_ON_ONCE

KVM: x86: Drop ASSERT() on I/O APIC EOIs being only for LEVEL_to WARN_ON_ONCE

Remove kvm_ioapic_update_eoi_one()'s ASSERT() that the vector's entry is
configured to be level-triggered, as KVM intercepts and forward EOIs to
the I/O APIC even for edge-triggered IRQs (see kvm_ioapic_scan_entry()),
and nothing guarantees the local APIC's TMR register is synchronized with
the I/O APIC redirection table, i.e. the @trigger_mode check just out of
sight doesn't provide any meaningful protection.

Given that roughly half of the historic ASSERT()s are/were guest- and/or
user-triggerable, it's safe to assume no one has run meaningful workloads
with DEBUG=1, i.e. that the ASSERT() has been dead code since it was
added 18+ years ago.

Opportunistically drop the unnecessary forward declaration of
kvm_ioapic_update_eoi_one().

For all intents and purposes, no functional change intended.

Link: https://patch.msgid.link/20251206004311.479939-4-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>