remove recommendation re: standard license headers
We have some text at the bottom of this page https://spdx.dev/ids/ regarding the use of SPDX ids related to a recommendation about using and retaining standard headers when using/adding an SPDX id in source code.
If memory serves, we wrote this at the time when use of SPDX ids in source code was a very new thing. We didn't know if some license stewards might have discomfort with the use of SPDX ids *instead* of their suggested standard license header, and thus felt the need to take a sort of conservative approach.
Now that SPDX ids are used more widely and we know a bit more about how scanning tools identify license headers in total - I think we can remove this section altogether. I don't think SPDX needs to make a statement either way and projects can make their own call, as we've seen with the Linux kernal and other projects.