native_functions.yaml: remove SparseXPU which is added by accident (#68791)
Summary:
gen_backend_stubs.py will report 'assert' when generate code with
SparseXPU dispatch key for external backends, if SparseXPU is in
native_functions.yaml.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/68791
Reviewed By: cpuhrsch, ejguan
Differential Revision: D32646303
Pulled By: bdhirsh
fbshipit-source-id: 64e42cc40468bc8c696a31b4b7c0cc3728866a64
