[BE] Remove unused `dim_plane` from `reflection_pad2d_backward_out_template` (#103680)
Probably introduced by https://github.com/pytorch/pytorch/pull/102254
This fixes `variable 'dim_plane' set but not used ` on my clang-14.0.3 compiler complained about it:
```
/Users/nshulga/git/pytorch/pytorch/aten/src/ATen/native/ReflectionPad.cpp:272:7: error: variable 'dim_plane' set but not used [-Werror,-Wunused-but-set-variable]
int dim_plane = 0;
^
1 error generated.
```
<!--
copilot:poem
-->
### <samp>🤖 Generated by Copilot at e254b4b</samp>
> _`dim_plane` is gone_
> _Simpler code, no more warning_
> _Autumn leaves fall fast_
Pull Request resolved: https://github.com/pytorch/pytorch/pull/103680
Approved by: https://github.com/kit1980, https://github.com/Skylion007
