removed not-always-applicable check for "polymorphic_identity"
removed not-always-applicable check for "polymorphic_identity"

elelianghh/sqlalchemy,bdupharm/sqlalchemy,EvaSDK/sqlalchemy,davidjb/sqlalchemy,epa/sqlalchemy,davidfraser/sqlalchemy,bootandy/sqlalchemy,pdufour/sqlalchemy,halfcrazy/sqlalchemy,zzzeek/sqlalchemy,Cito/sqlalchemy,wujuguang/sqlalchemy,Cito/sqlalchemy,wujuguang/sqlalchemy,WinterNis/sqlalchemy,alex/sqlalchemy,WinterNis/sqlalchemy,sandan/sqlalchemy,ThiefMaster/sqlalchemy,EvaSDK/sqlalchemy,wfxiang08/sqlalchemy,ThiefMaster/sqlalchemy,sqlalchemy/sqlalchemy,dstufft/sqlalchemy,monetate/sqlalchemy,itkovian/sqlalchemy,davidfraser/sqlalchemy,brianv0/sqlalchemy,davidjb/sqlalchemy,robin900/sqlalchemy,itkovian/sqlalchemy,dstufft/sqlalchemy,alex/sqlalchemy,brianv0/sqlalchemy,sandan/sqlalchemy,Akrog/sqlalchemy,bdupharm/sqlalchemy,j5int/sqlalchemy,halfcrazy/sqlalchemy,276361270/sqlalchemy,Akrog/sqlalchemy,epa/sqlalchemy,elelianghh/sqlalchemy,wfxiang08/sqlalchemy,alex/sqlalchemy,bootandy/sqlalchemy,graingert/sqlalchemy,276361270/sqlalchemy,olemis/sqlalchemy,Cito/sqlalchemy,hsum/sqlalchemy,hsum/sqlalchemy,graingert/sqlalchemy,robin900/sqlalchemy,inspirehep/sqlalchemy,inspirehep/sqlalchemy,monetate/sqlalchemy,j5int/sqlalchemy,olemis/sqlalchemy,pdufour/sqlalchemy