Remove StringNetworkSpecifier

Due to issues with @Deprecated @removed, manually remove
StringNetworkSpecifier. This API was never in any released
API surface; it was planned to be added in R; however, the
need was superseded by TelephonyNetworkSpecifier.

"Aaaaand it's gone!"

Bug: 147441267
Test: make aosp_bramble-userdebug
Change-Id: I9bae68a60f3cdb77203ba61ea8a08f64e597cf08