THRIFT-2770: D: thrift.internal.traits unittest fails due to std.metastrings import

I originally wrote what became std.traits.SetFunctionAttributes
as thrift.internal.traits.ChangeFuncAttrs. Now that it has been
in Phobos for a number of releases, remove the local copy.

This is both a cleanup and fixes the -unittest build in 2.066,
where std.metastrings (which was just a stray import anyway)
has finally been removed.