Remove "struct" markers from varlena, varatt_external and varatt_indirect

Hi all,

"struct" markers are not the style of the project for variable
declarations, and we have been historically applying these for various
declarations in the code for varlena, varatt_external and
varatt_indirect, so I would like to propose the attached patch to
clean up all this code.

This has been extracted from a larger patch I have sent a couple of
months ago, worth a cleanup of its own now that I am looking at it
again. For reference, this has also been proposed by Tom here (see
around 0003, but the varlena bits are my own):
https://www.postgresql.org/message-id/1891064.1754681536@sss.pgh.pa.us

This cleanup leads to the attached, with the CI not complaining. That
will also help me a bit for the other patch sets I have been playing
with related to TOAST.

Thoughts or comments?
--
Michael