Add Link header handling to ThermalSubsystem

Along with adding Link header, this commit updates ThermalSubsystem
to respond to HEAD requests. The link header is something required
by the Redfish spec.

Also, Removed redundant code[1]

[1] https://gerrit.openbmc.org/c/openbmc/bmcweb/+/58731/2/redfish-core/lib/thermal_subsystem.hpp#b48



Tested:
HEAD /redfish/v1/Chassis/<str>/ThermalSubsystem returns a correct
looking Link header.
Following that Link header resolves the schema file.

Get /redfish/v1/Chassis/BadChassis/ThermalSubsystem returns 404.

Signed-off-by: George Liu <liuxiwei@inspur.com>
Change-Id: Ib93573808e4370ac45c2051517cd3f1c28e0f8bb