x86: vmx: Remove max_index tracking in check_vmcs_field()

Now that comprehensive search for maximum VMCS field index is performed,
the tracking of the maximum index in __check_vmcs_field() is no longer
needed. Remove all the related logic accordingly.

Cc: Jim Mattson <jmattson@google.com>
Cc: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Nadav Amit <namit@vmware.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>