Undo unintentional ABI break in struct ResultRelInfo.

Undo unintentional ABI break in struct ResultRelInfo.

Commits aac2c9b4f et al. added a bool field to struct ResultRelInfo.
That's no problem in the master branch, but in released branches
care must be taken when modifying publicly-visible structs to avoid
an ABI break for extensions.  Frequently we solve that by adding the
new field at the end of the struct, and that's what was done here.
But ResultRelInfo has stricter constraints than just about any other
node type in Postgres.  Some executor APIs require extensions to index
into arrays of ResultRelInfo, which means that any change whatever in
sizeof(ResultRelInfo) causes a fatal ABI break.

Fortunately, this is easy to fix, because the new field can be
squeezed into available padding space instead --- indeed, that's where
it was put in master, so this fix also removes a cross-branch coding
variation.

Per report from Pavan Deolasee.  Patch v14-v17 only; earlier versions
did not gain the extra field, nor is there any problem in master.

Discussion: https://postgr.es/m/CABOikdNmVBC1LL6pY26dyxAS2f+gLZvTsNt=2XbcyG7WxXVBBQ@mail.gmail.com