doc: remove obsolete deprecation notice for power library

doc: remove obsolete deprecation notice for power library

Remove notice announcing an already-implemented change.

In 19.05, rte_power_set_env was changed to return -1 in cases where
the environment was already set up, and for the same release, a
deprecation notice was added.
This patch removes that notice.

The API change was tested by calling rte_power_set_env twice. The first
call succeeded, and the second call failed, as expected.

Fixes: 5a5f3178d4a8 ("power: return error when environment already set")
Cc: stable@dpdk.org
Signed-off-by: David Hunt <david.hunt@intel.com>