Revert "Track statistics for spilling of changes from ReorderBuffer".

Revert "Track statistics for spilling of changes from ReorderBuffer".

The stats with this commit was available only for WALSenders, however,
users might want to see for backends doing logical decoding via SQL API.
Then, users might want to reset and access these stats across server
restart which was not possible with the current patch.

List of commits reverted:

caa3c4242c   Don't call elog() while holding spinlock.
e641b2a995   Doc: Update the documentation for spilled transaction
statistics.
5883f5fe27   Fix unportable printf format introduced in commit 9290ad198.
9290ad198b   Track statistics for spilling of changes from ReorderBuffer.

Additionaly, remove the release notes entry for this feature.

Backpatch-through: 13, where it was introduced
Discussion: https://postgr.es/m/CA+fd4k5_pPAYRTDrO2PbtTOe0eHQpBvuqmCr8ic39uTNmR49Eg@mail.gmail.com