Remove duplicate code from ReorderBufferCleanupTXN().

Remove duplicate code from ReorderBufferCleanupTXN().

Andres is apparently the only hacker who thinks this code is better as-is.
I (tgl) follow some of his logic, but the fact that it's setting off
warnings from static code analyzers seems like a sufficient reason to
put the complexity into a comment rather than the code.

Aleksander Alekseev

Discussion: <20160404190345.54d84ee8@fujitsu>