At least with one Clang build, executing sw_unoapi test aborts at shutdown of
soffice.bin with

20 __cxxabiv1::__cxa_call_unexpected
21 XMLRedlineImportHelper::~XMLRedlineImportHelper
22 XMLRedlineImportHelper::~XMLRedlineImportHelper
23 SwXMLTextImportHelper::~SwXMLTextImportHelper
24 SwXMLTextImportHelper::~SwXMLTextImportHelper
25 UniRefBase::release
26 UniReference<XMLTextImportHelper>::~UniReference
27 UniReference<XMLTextImportHelper>::~UniReference
28 SvXMLImport::~SvXMLImport
29 SwXMLImport::~SwXMLImport
30 SwXMLImport::~SwXMLImport
31 cppu::OWeakObject::release
[...]
79 binaryurp::Bridge::dispose
[...]
98 desktop::Desktop::DeregisterServices
99 desktop::Desktop::doShutdown
100 desktop::Desktop::Main

due to ~XMLRedlineImportHelper calling setPropertyValue on an SwXTextDocument
that is !IsValid() and thus throws a RuntimeException.  SwXMLImport::endOocument
already contains ClearShapeImport with a comment that indicates it is necessary
for a similar reason, so add a ClearTextImport that ensures any
~XMLRedlineImportHelper activity is done before the SwXTextDocument becomes
invalid, and not only at shutdown of the remote bridge that still holds a (Java)
reference to SwXMLImport.

Change-Id: I9891879c91bfea79fa9e22691be1826f61fdd70a