Simplify some log messages in extended_stats_funcs.c

Simplify some log messages in extended_stats_funcs.c

The log messages used in this file applied too much quoting logic:
- No need for quote_identifier(), which is fine to not use in the
context of a log entry.
- The usual project style is to group the namespace and object together
in a quoted string, when mentioned in an log message.  This code quoted
the namespace name and the extended statistics object name separately,
which was confusing.

Reported-by: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Discussion: https://postgr.es/m/20260210.143752.1113524465620875233.horikyota.ntt@gmail.com