Cleanup code in reloptions.h regarding reloption handling

Cleanup code in reloptions.h regarding reloption handling

reloptions.h includes since ba748f7 a set of macros to handle reloption
types in a way similar to how parseRelOptions() works.  They have never
been used in the core code, and we have more simple methods now to parse
and fill in rd_options for a given relation depending on its relkind, so
remove this interface to simplify things.

Per discussion between Amit Langote, Álvaro Herrera and me.

Discussion: https://postgr.es/m/CA+HiwqE6zbNO92az6pp5GiTw4tr-9rfCE0t84whQSP+YwSKjMQ@mail.gmail.com