Remove unused #include's from contrib, pl, test .c files
as determined by IWYU
Similar to commit
dbbca2cf299, but for contrib, pl, and src/test/.
Reviewed-by: Alvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://www.postgresql.org/message-id/flat/
0df1d5b1-8ca8-4f84-93be-
121081bde049%40eisentraut.org