pageinspect: Make page deletion elog less chatty.

pageinspect: Make page deletion elog less chatty.

An elog that reports the value of a transaction ID stored on a deleted
nbtree page was added by commit e5d8a999, which taught page deletion to
store full 64-bit XIDs.  It seems very chatty on further reflection, so
lower its elevel from NOTICE to DEBUG2.

Author: Peter Geoghegan <pg@bowt.ie>
Backpatch: 14-, just like the nbtree XID enhancement.