pgsql: 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(dot)ntt(at)gmail(dot)com>
Discussion: https://postgr.es/m/20260210.143752.1113524465620875233.horikyota.ntt@gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f33c585774223757b01c8eddd134d364492ed94c

Modified Files
--------------
src/backend/statistics/extended_stats_funcs.c | 32 +++++++++++----------------
src/test/regress/expected/stats_import.out | 14 ++++++------
2 files changed, 20 insertions(+), 26 deletions(-)