This reverts commit cd2725de.

Norbert's list of CI performance issues includes
"2/ clang plugin build should not be serialized (forced -j1)"

Change-Id: Ib77f951a31adc20f6a9f88e8b51632bd81273327
Reviewed-on: https://gerrit.libreoffice.org/42595Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Stahl <mstahl@redhat.com>