x86: kvm: vmx: Remove some unused functions

Removes some functions that are not used anywhere:
cpu_has_vmx_eptp_writeback() cpu_has_vmx_eptp_uncacheable()

This was partially found by using a static code analysis program called cppcheck.

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>