vendor: apns: Remove unused "spn" attribute

* The parsing of apns-conf.xml happens in
  packages/providers/TelephonyProvider/src/com/android/providers/telephony/
  TelephonyProvider.java
* Looking at the code, the only possible occurance of "spn" is as a
  value for "mvno_type"
  -> Remove all spn="" attributes

Change-Id: Ic7fef88c98ad54ea98d702e028a0455de22e55cd