Replace redundant boolean literal in conditional return statement in variable.cpp.
Replace redundant boolean literal in conditional return statement in variable.cpp.

hsorby/libcellml,cellml/libcellml,cellml/libcellml,hsorby/libcellml,nickerso/libcellml,hsorby/libcellml,MichaelClerx/libcellml,nickerso/libcellml,MichaelClerx/libcellml,nickerso/libcellml,nickerso/libcellml,hsorby/libcellml,MichaelClerx/libcellml,cellml/libcellml,cellml/libcellml