Remove unused typedefs

Remove unused typedefs

There were a few typedefs that were never used to define a variable or
field.  This had the effect that the buildfarm's typedefs.list would
not include them, and so they would need to be re-added manually to
keep the overall pgindent result perfectly clean.

We can easily get rid of these typedefs to avoid the issue.  In a few
cases, we just let the enum or struct stand on its own without a
typedef around it.  In one case, an enum was abused to define flag
bits; that's better done with macro definitions.

This fixes all the remaining issues with missing entries in the
buildfarm's typedefs.list.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/1919000.1715815925@sss.pgh.pa.us