Remove op_linear_scratch_example

Summary:
Example file that is deprecated. Removing as it has a security vulnerability found by the fuzzer

Heap buffer overflow in [linear_scratch_example()](https://www.internalfb.com/code/fbsource/[9a6a9cfa7eab]/fbcode/executorch/kernels/portable/cpu/op_linear_scratch_example.cpp?lines=102)

Differential Revision: D88285745

Author: lucylq

kernels/portable/cpu/op_linear_scratch_example.cpp

@@ -30,8 +30,3 @@
   kernels:
     - arg_meta: null
       kernel_name: torch::executor::allclose_tensor
-
-- func: linear.scratch_example(Tensor input, Tensor weight, Tensor? bias=None, *, Tensor(a!) out, Tensor(b!) _scratch_tensor) -> Tensor(a!)
-  kernels:
-    - arg_meta: null
-      kernel_name: torch::executor::linear_scratch_example

kernels/portable/custom_ops.yaml

@@ -140,7 +140,6 @@ PORTABLE_KERNELS_SRCS = [
     "kernels/portable/cpu/op_le.cpp",
     "kernels/portable/cpu/op_leaky_relu.cpp",
     "kernels/portable/cpu/op_lift_fresh_copy.cpp",
-    "kernels/portable/cpu/op_linear_scratch_example.cpp",
     "kernels/portable/cpu/op_log.cpp",
     "kernels/portable/cpu/op_log10.cpp",
     "kernels/portable/cpu/op_log1p.cpp",

shim_et/xplat/executorch/build/build_variables.bzl

@@ -1367,9 +1367,6 @@ CUSTOM_OPS = (
     op_target(
         name = "op_allclose",
     ),
-    op_target(
-        name = "op_linear_scratch_example",
-    ),
 )
 
 def portable_source_list():