runtime-spec: Linux: Don't mount /sys and /proc (obsolete)

The mount-requirement was softened to a SHOULD in [1].  It's not clear
to me whether that SHOULD is directed at config authors ("you should
explicitly include mounts for these") or at runtimes ("you should
provide these even if the config doesn't ask for them"), but my
attempts to clarify that one way or the other were both rejected
[2,3].  The current runtime-tools and runC approach favors the
config-author direction [4], which is what I'd asked for in the
original thread post, so I'm tagging this obsolete.

[1]: opencontainers/runtime-spec#666
[2]: opencontainers/runtime-spec#679 (comment)
[3]: opencontainers/runtime-spec#678 (comment)
[4]: opencontainers/runtime-tools#24