MINOR: fix cleanup phase for KStreamWindowAggregateTest

fixes:
```
java.nio.file.NoSuchFileException: /tmp/test7863510415433793941/topic2-Canonized/topic2-Canonized-197001010000/000015.sst
	at sun.nio.fs.UnixException.translateToIOException(UnixException.java:86)
	at sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:102)
	at sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:107)
	at sun.nio.fs.UnixFileAttributeViews$Basic.readAttributes(UnixFileAttributeViews.java:55)
	at sun.nio.fs.UnixFileSystemProvider.readAttributes(UnixFileSystemProvider.java:144)
	at sun.nio.fs.LinuxFileSystemProvider.readAttributes(LinuxFileSystemProvider.java:97)
	at java.nio.file.Files.readAttributes(Files.java:1686)
	at java.nio.file.FileTreeWalker.walk(FileTreeWalker.java:105)
	at java.nio.file.FileTreeWalker.walk(FileTreeWalker.java:199)
	at java.nio.file.FileTreeWalker.walk(FileTreeWalker.java:199)
	at java.nio.file.FileTreeWalker.walk(FileTreeWalker.java:199)
	at java.nio.file.FileTreeWalker.walk(FileTreeWalker.java:69)
	at java.nio.file.Files.walkFileTree(Files.java:2602)
	at java.nio.file.Files.walkFileTree(Files.java:2635)
	at org.apache.kafka.common.utils.Utils.delete(Utils.java:555)
	at org.apache.kafka.streams.kstream.internals.KStreamWindowAggregateTest.testJoin(KStreamWindowAggregateTest.java:320)
```

Author: Matthias J. Sax <[email protected]>

Reviewers: Eno Thereska <[email protected]>, Damian Guy <[email protected]>, Jun Rao <[email protected]>

Closes apache#2778 from mjsax/minor-fix-kstreamWindowAggregateTest