redfish: thermal: Remove TODO, fix error msg

This commit makes two minor cleanups to the Thermal resource:

1. Removes an obsolete `TODO` comment regarding Chassis Redundancy
   information in the GET handler. The redundancy data is already
   being correctly populated for Thermal resources via the
   `populateFanRedundancy()` function called within `sensors.hpp`.

2. Fixes a copy-paste error in the PATCH handler. If an empty payload
   was provided, the `resourceNotFound` message incorrectly referenced
   "Voltages" (which belongs to the Power schema) instead of "Fans".

Change-Id: Icb35f1e85fe56895dcc47c6241a556da3a3238d9
Signed-off-by: Shivraj Toravi <shivrajnt@ami.com>