KVM: VMX: remove duplicated declaration

Declaration of VMX_VPID_EXTENT_SUPPORTED_MASK occures twice in the code.
Probably, it was happened after unsuccessful merge.

Signed-off-by: Jan Dakinevich <jan.dakinevich@gmail.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>