The argsize PCDATA was specifying the number of
bytes passed to a function call, so that if the function
did not specify its argument count, the garbage collector
could use the call site information to scan those bytes
conservatively. We don't do that anymore, so stop
generating the information.

LGTM=khr
R=khr
CC=golang-codereviews
https://golang.org/cl/139530043