Refactor length calculation (#2322)
Refactor length calculation (#2322)

Also, avoid setting an unnecessary field on the album objects. :/

sampsyo/beets,Kraymer/beets,lengtche/beets,diego-plan9/beets,artemutin/beets,artemutin/beets,artemutin/beets,mosesfistos1/beetbox,pkess/beets,lengtche/beets,shamangeorge/beets,xsteadfastx/beets,diego-plan9/beets,xsteadfastx/beets,mosesfistos1/beetbox,beetbox/beets,beetbox/beets,MyTunesFreeMusic/privacy-policy,mosesfistos1/beetbox,shamangeorge/beets,pkess/beets,mosesfistos1/beetbox,jackwilsdon/beets,MyTunesFreeMusic/privacy-policy,SusannaMaria/beets,beetbox/beets,madmouser1/beets,madmouser1/beets,sampsyo/beets,madmouser1/beets,diego-plan9/beets,xsteadfastx/beets,Kraymer/beets,xsteadfastx/beets,shamangeorge/beets,lengtche/beets,pkess/beets,ibmibmibm/beets,artemutin/beets,shamangeorge/beets,Kraymer/beets,jackwilsdon/beets,pkess/beets,Kraymer/beets,madmouser1/beets,sampsyo/beets,SusannaMaria/beets,SusannaMaria/beets,jackwilsdon/beets,SusannaMaria/beets,ibmibmibm/beets,MyTunesFreeMusic/privacy-policy,jackwilsdon/beets,lengtche/beets,ibmibmibm/beets,sampsyo/beets,MyTunesFreeMusic/privacy-policy,ibmibmibm/beets,diego-plan9/beets,beetbox/beets