Remove false comment about speculative insertion.
Remove false comment about speculative insertion.

There is no full discussion of speculative insertions in the executor
README. There is a high-level explanation in execIndexing.c, but it doesn't
seem necessary to refer it from here.

Peter Geoghegan

zeroae/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl