[network] Do not check the service table before reading SPDI

Although it is correct to check support for service provider display
info from service table before reading, operators don't necessarily
care about such details. Direct reading is the surest way to find out
if the info is present or not. If it is not, ofono will just get
failure from modem.

Signed-off-by: Jussi Kangas <jussi.kangas@oss.tieto.com>