pgsql: Clean up date_part tests a bit

Clean up date_part tests a bit

Some tests for timestamp and timestamptz were in the date.sql test
file. Move them to their appropriate files, or drop tests cases that
were already present there.

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/efcc7572f532ea564fedc6359c2df43045ee7908

Modified Files
--------------
src/test/regress/expected/date.out | 80 +----------------
src/test/regress/expected/timestamp.out | 137 ++++++++++++++---------------
src/test/regress/expected/timestamptz.out | 139 +++++++++++++++---------------
src/test/regress/sql/date.sql | 20 +----
src/test/regress/sql/timestamp.sql | 3 +-
src/test/regress/sql/timestamptz.sql | 3 +-
6 files changed, 147 insertions(+), 235 deletions(-)