Drop warning-free support for Flex 2.5.35

Drop warning-free support for Flex 2.5.35

This removes all the various workarounds for avoiding compiler
warnings with Flex 2.5.35.  Several recent patches have added
additional warnings that would either need to be fixed along the lines
of the existing workarounds, or we decide to no longer care about
this, which we do here.

Flex 2.5.35 is extremely outdated, and you can't even download it
anymore from any of the Flex project sites, so it's nearly impossible
to support.

After this, using Flex 2.5.35 will still work, but the generated code
will produce numerous compiler warnings.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/1a204ccd-7ae6-478c-a431-407b5c48ccc6@eisentraut.org