libcore: removed unnecessary code in SSLSocketTest

Was possibly causing a race condition (see b/21776011),
for instance, in case readFirstReceivedChunkFuture
was still executing IoUtils#closeQuietly while the
thread was cancelled.

Bug: 21776011
Change-Id: Id3d1c7825aae393f30b586e3c9a7200b367cfaa8
1 file changed