drm/xe/guc: Remove explicit shutdown of SLPC

SLPC shutdown is called in reset and suspend paths. In the reset
path, it is possible that the H2G call gets lost as GuC is in the
process of being reset. There is no value in stopping SLPC when
it will happen anyways.

In the suspend path, we disable communication with GuC, so there
is no need to explicitly shutdown SLPC.

v2: Rebase

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240325235602.1155486-1-vinay.belgaumkar@intel.com