Drop FSEND_t: C-style varargs code was replaced since version 6.0 in commit
d43fc407, so it's time to remove the last bits.

Drop int-to-string macros that are simple wrappers over OString::number.

Avoid unnecessary string type conversions.

Change-Id: I86478577b8e469f99b5a90619e5f78a61f4f23fb
Reviewed-on: https://gerrit.libreoffice.org/70964
Tested-by: Jenkins
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>