KVM: x86: Remove kvm_vcpu_is_illegal_gpa()

KVM: x86: Remove kvm_vcpu_is_illegal_gpa()

Remove kvm_vcpu_is_illegal_gpa() and use !kvm_vcpu_is_legal_gpa() instead.
The "illegal" helper actually predates the "legal" helper, the only reason
the "illegal" variant wasn't removed by commit 4bda0e97868a ("KVM: x86:
Add a helper to check for a legal GPA") was to avoid code churn.  Now that
CR3 has a dedicated helper, there are fewer callers, and so the code churn
isn't that much of a deterrent.

No functional change intended.

Signed-off-by: Binbin Wu <binbin.wu@linux.intel.com>
Tested-by: Xuelian Guo <xuelian.guo@intel.com>
Link: https://lore.kernel.org/r/20230913124227.12574-8-binbin.wu@linux.intel.com
[sean: provide a bit of history in the changelog]
Signed-off-by: Sean Christopherson <seanjc@google.com>