Delete JarFileTest.testGetJarEntry().

The first five lines of this test duplicate assertions already
present in other tests.

The rest of the test attempts to download
http://jcltest.apache.org/testres231/../internalres/signedjars/jarlist.txt
the host of which no longer exists. Usually this triggers an IOException,
which is caught in the test and causes the test to do nothing. However it
can also lead to timeouts during the DNS lookup, which makes the test
flaky (has been observed to fail with timeout under vogar).

This is not the only test that relies on a third party server; the others
are left to future CLs.

This CL drops the flaky and redundant/nonfunctional test.

Bug: 78269207
Bug: 78278018
Test: Treehugger
Change-Id: If2baa5d7dc00b899a46422342432f49303ea6d2a