KVM: arm64: Drop unused workaround_flags vcpu field

workaround_flags is a leftover from our earlier Spectre-v4 workaround
implementation, and now serves no purpose.

Get rid of the field and the corresponding asm-offset definition.

Fixes: 29e8910a ("KVM: arm64: Simplify handling of ARCH_WORKAROUND_2")
Signed-off-by: Marc Zyngier <maz@kernel.org>