Revert "Allow on-line enabling and disabling of data checksums"

This reverts the backend sides of commit 1fde38beaa.
I have, at least for now, left the pg_verify_checksums tool in place, as
this tool can be very valuable without the rest of the patch as well,
and since it's a read-only tool that only runs when the cluster is down
it should be a lot safer.

doc/src/sgml/reference.sgml

@@ -284,7 +284,6 @@
    &pgtestfsync;
    &pgtesttiming;
    &pgupgrade;
-   &pgVerifyChecksums;
    &pgwaldump;
    &postgres;
    &postmaster;