Bug 451888 - Update maven-clean-plugin used to 2.6.1 .

Build of swt.binaries has a special cleanup task which is set to use
maven-clean-plugin 2.5. Updating it to 2.6.1 gives us the following
fixed and performance improvements
http://jira.codehaus.org/secure/ReleaseNote.jspa?projectId=11128&version=20685
and
http://jira.codehaus.org/secure/ReleaseNote.jspa?projectId=11128&version=18533
.

Change-Id: I02fa1bde10b627571dc2c23acf3a2ad9acf0f5f3
Signed-off-by: Alexander Kurtakov <akurtako@redhat.com>