Drop processYield return type.

The virtual Yield function has no return type, and the result of
processYield is actually never used => drop it.

Change-Id: I3da7bc99fd8f56301da37ca196b03b34cdc84a40
Reviewed-on: https://gerrit.libreoffice.org/6684
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>