Remove spurious WTF in rematching networks.

By construction, this WTF should never happen, since it's in an
if (nri.request.isRequest()) and by definition requests can only
be satisfied by one network at a time.

I don't think we've ever seen this particular WTF in an APR
report, which suggests that it's not happening in practice.

Test: atest FrameworksNetTests CtsNetTestCasesLegacyApi22 CtsNetTestCasesLegacyPermission22 android.net.cts.ConnectivityManagerTest
Change-Id: Icf4c7d2bb1da3c7db695cf0bcebc5806190a1677