runtime: rename SchedType to SchedT

CL 144940043 renamed it from Sched to SchedType
to avoid a lowercasing conflict in the Go code with
the variable named sched.
We've been using just T resolve those conflicts, not Type.

The FooType pattern is already taken for the kind-specific
variants of the runtime Type structure: ChanType, MapType,
and so on. SchedType isn't a Type.

LGTM=bradfitz, khr
R=khr, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/145180043