Remove NetworkShimImpl mock in TcpSocketTrackerTest

Lint flags the usage of NetworkShimImpl in TcpSocketTrackerTest:
packages/modules/NetworkStack/tests/unit/src/com/android/networkstack/netlink/TcpSocketTrackerTest.java:234: Error: Class requires API level S (current min is 29): NetworkShimImpl [NewApi]
                .spyStatic(NetworkShimImpl.class)
                           ~~~~~~~~~~~~~~~~~~~~~

Remove the NetworkShimImpl mock that is not used.

Test: atest TcpSocketTrackerTest
Test: m lint-check
Change-Id: Ifd8994e36bb024d5ea79c4cdb8a99faa0a1472b7