stress: Remove essentially unused library.

stress: Remove essentially unused library.

The "stress" library was introduced years ago.  We intended at the time to
start using it to provoke errors in testing, to make sure that Open vSwitch
was resilient against those errors.  The intention was good, but there were
few actual implementations of stress options, and the testing never
materialized.

Rather than adapt the stress library for thread safety, this seems like a
good opportunity to remove it, so this commit does so.

Signed-off-by: Ben Pfaff <blp@nicira.com>