Remove a base_unit attribute missed from previous commit.
Remove a base_unit attribute missed from previous commit.

hsorby/libcellml,cellml/libcellml,MichaelClerx/libcellml,MichaelClerx/libcellml,nickerso/libcellml,cellml/libcellml,cellml/libcellml,cellml/libcellml,libcellml/buildbot-testing,hsorby/libcellml,nickerso/libcellml,libcellml/buildbot-testing,nickerso/libcellml,hsorby/libcellml,nickerso/libcellml,hsorby/libcellml,MichaelClerx/libcellml