Remove unused section template
Remove unused section template

Summary:
- Remove the `.section` template at the top of `buckconfig.soy` which is no longer used.
- This template is a liability because it can easily be confused for the `.section` template defined in `__buckconfig_common.soy`, which is the template that is actually used to implement the sections in the buckconfig docs.

Reviewed By: styurin

fbshipit-source-id: 743f0446d0

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