Bug 1009961 - Remove code to drop displayport if it exceeds B2G max texture size. r=tn

This code is no longer needed because tiling is enabled on all
scrollable layers on B2G, and so we only draw tile-sized chunks
of the displayport. Therefore the textures created will never
exceed the max texture size. On non-B2G platforms this code is
not used anyway so there is no effect. Going forward we should
always be enabling tiling before enabling APZ on new platforms
so it is unlikely that we will need this code.