Remove too-strict filestore available size test

[dbwiddis]

The test that usable space equals available space on a filestore is too strict, as it can fail in a race condition between fetching statvfs and getting the same information from core Java:

assertEquals(fstore.getUsableSpace(), vfs.f_bavail.longValue() * vfs.f_bsize.longValue());

Failed test:

    [junit] Testcase: testStatvfs(com.sun.jna.platform.linux.LibCTest):	FAILED
    [junit] expected:<816184561664> but was:<816175988736>

In theory we could test for something "close" but any change threshold we choose would be arbitrary. Given the previous line checks equality of total space, which should not change, I think this test can simply be removed.

[matthiasblaesing]

Sounds sane.