Remove ext-mbstring dependency.
Remove ext-mbstring dependency.

mbstring is not a hard dependency for 2.x. The Multibyte class can shim
up the missing extension.

Refs #2564

gmponos/cakephp,ndm2/cakephp,berrygoudswaard/cakephp,Phillaf/cakephp,CVO-Technologies/cakephp,dakota/cakephp,garethellis36/cakephp,erikfig/cakephp,cakephp/cakephp,tenkoma/cakephp,tgr0ss/cakephp,dkarvounaris/cakephp,timhoeppner/cakephp,tisu2tisu/cakephp,et304383/passbolt_api,CakeDC/cakephp,bravo-kernel/cakephp,CakeDC/cakephp,khoimt/cakephp-study,elboletaire/cakephp,thinkingmedia/cakephp,jjj117/cakephp,CakeDC/cakephp,akintoey/cakephp,gmponos/cakephp,bravo-kernel/cakephp,pbaire/cakephp,dkarvounaris/cakephp,mvdriel/cakephp,ionas/cakephp,jerems02/cakephp,ndm2/cakephp,timhoeppner/cakephp,ionas/cakephp,Phillaf/cakephp,davidyell/cakephp,burriko/cakephp,jeffblack360/cakephp,nrother/cakephp,ndm2/cakephp,jeffblack360/cakephp,petteyg/cakephp,kumamidori/cakephp,akintoey/cakephp,davidyell/cakephp,garethellis36/cakephp,web-player/cakephp,ADmad/cakephp,dkarvounaris/cakephp,gmponos/cakephp,khoimt/cakephp-study,mvdriel/cakephp,akintoey/cakephp,HavokInspiration/cakephp,CakeDC/cakephp,mvdriel/cakephp,adopico/cakephp,HavokInspiration/cakephp,michaelze/cakephp,kumamidori/cakephp,jjj117/cakephp,jerems02/cakephp,erikfig/cakephp,Phillaf/cakephp,tgr0ss/cakephp,ADmad/cakephp,eryw/cakephp,elboletaire/cakephp,cakephp-fr/cakephp,ADmad/cakephp,Balachan27/cakephp,thinkingmedia/cakephp,erikfig/cakephp,web-player/cakephp,elboletaire/cakephp,timhoeppner/cakephp,michaelze/cakephp,eboominathan/cakephp,tisu2tisu/cakephp,justinyost/cakephp,tisu2tisu/cakephp,eryw/cakephp,pbaire/cakephp,burriko/cakephp,jeffblack360/cakephp,tenkoma/cakephp,eboominathan/cakephp,CVO-Technologies/cakephp,web-player/cakephp,berrygoudswaard/cakephp,tenkoma/cakephp,jjj117/cakephp,kumamidori/cakephp,adopico/cakephp,Balachan27/cakephp,jerems02/cakephp,ndm2/cakephp,pbaire/cakephp,berrygoudswaard/cakephp,adopico/cakephp,ionas/cakephp,yinhe007/cakephp,eryw/cakephp,ADmad/cakephp,justinyost/cakephp,CVO-Technologies/cakephp,justinyost/cakephp,dakota/cakephp,burriko/cakephp,tgr0ss/cakephp,tenkoma/cakephp,nrother/cakephp,garethellis36/cakephp,cakephp-fr/cakephp,et304383/passbolt_api,davidyell/cakephp,yinhe007/cakephp,yinhe007/cakephp,michaelze/cakephp,cakephp-fr/cakephp,cakephp/cakephp,dakota/cakephp,garethellis36/cakephp,nrother/cakephp,mvdriel/cakephp,Balachan27/cakephp,khoimt/cakephp-study,petteyg/cakephp,dakota/cakephp,cakephp/cakephp,et304383/passbolt_api,eboominathan/cakephp,petteyg/cakephp,et304383/passbolt_api,thinkingmedia/cakephp,et304383/passbolt_api,cakephp/cakephp,HavokInspiration/cakephp,petteyg/cakephp,bravo-kernel/cakephp