stop cleaning mw/core workspace to save I/O

So we have the Git plugin to wipe the workspace and reclone the git
repository on any job/change. That must add a lot of I/O stress on the
continuous integration server.

This change disable workspace wipeout and use the git clean -fdx
instead.

XML changed:

-    <clean>false</clean>
-    <wipeOutWorkspace>true</wipeOutWorkspace>
+    <clean>true</clean>
+    <wipeOutWorkspace>false</wipeOutWorkspace>

Jobs impacted:

mediawiki-core-doxygen-publish
mediawiki-core-install-sqlite
mediawiki-core-jsduck
mediawiki-core-jsduck-publish
mediawiki-core-jslint
mediawiki-core-lint
mediawiki-core-merge
mediawiki-core-phpunit-api
mediawiki-core-phpunit-databaseless
mediawiki-core-phpunit-misc
mediawiki-core-phpunit-parser
mediawiki-core-qunit
mediawiki-core-regression-REL1_19
mediawiki-core-regression-REL1_20
mediawiki-core-regression-REL1_21
mediawiki-core-regression-master
mediawiki-core-whitespaces
test-mediawiki-qunit

Change-Id: Icba850d433b8e116caed7e13d0f7daa5afc6cb79