Remove traces of support for Sun Studio compiler

Remove traces of support for Sun Studio compiler

Per discussion, this compiler suite is no longer maintained, and
it has not been able to compile PostgreSQL since at least PostgreSQL
17.

This removes all the remaining support code for this compiler.

Note that the Solaris operating system continues to be supported, but
using GCC as the compiler.

Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/a0f817ee-fb86-483a-8a14-b6f7f5991b6e%40eisentraut.org