Remove unused `extensions_info_file` property from framework-doc-stubs

The `droidstubs.extensions_info_file` property is only used when
`api_levels_annotations_enabled: true`. The `framwork-doc-stubs` uses
`api_levels_module` instead and so does not need `extensions_info_file`
any more.

Bug: 305688818
Test: m framework-doc-stubs
      # Before and after and check output to confirm that it does not change
Change-Id: I4ad3f5b24e998eb9d5d47fa2240d2281fa9d6fc6