x86/sgx: Drop 'page_index' from sgx_backing

Storing the 'page_index' value in the sgx_backing struct is
dead code and no longer needed.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Kristen Carlson Accardi <kristen@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lkml.kernel.org/r/20220708162124.8442-1-kristen@linux.intel.com