Removed unused AspNet.Net35 Net35Backport

This backport class became obsolete after fixing the PreBodyTagFilter
with pull request #456
Removed reference to deleted file from NuSpec

Glimpse.AspNet.nuspec contained a reference towards the deleted
Glimpse.AspNet.Net35\Backport\Net35Backport.cs file
Merge pull request #458 from Glimpse/RemoveNet35BackportBecauseUnused

Removed unused AspNet.Net35 Net35Backport