KVM: SVM: Remove outdated comment in svm_load_mmu_pgd()

The comment had been added in the commit 689f3bf2 ("KVM: x86: unify
callbacks to load paging root") and its related code was removed later,
and it has nothing to do with the next line of code.

So the comment should be removed too.

Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
Message-Id: <20211108124407.12187-8-jiangshanlai@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>