syscall: remove nacl srpc helper

Fixes #11961

Minux removed the use of SRPC in 003dccfa, but the SRPC name service
code was left in the tree. SRPC was removed in pepper_42 making the
code, which ran on startup, fail, even though it was not used.

Removing srpc_nacl.go for a total diff of -822 lines has got to count
as one of the easiest nacl fixes we've had to date.

Change-Id: Ic4e348146bfe47450bbb9cabb91699ba153e6bf0
Reviewed-on: https://go-review.googlesource.com/13958Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>
Run-TryBot: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>