pgsql: Remove the useless and rather inconsistent return values of

Log Message:
-----------
Remove the useless and rather inconsistent return values of EncodeDateOnly,
EncodeTimeOnly, EncodeDateTime, EncodeInterval.  These don't have any good
reason to fail, and their callers were mostly not checking anyway.

Modified Files:
--------------
    pgsql/src/backend/utils/adt:
        datetime.c (r1.204 -> r1.205)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/datetime.c?r1=1.204&r2=1.205)
        timestamp.c (r1.198 -> r1.199)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/timestamp.c?r1=1.198&r2=1.199)
    pgsql/src/include/utils:
        datetime.h (r1.72 -> r1.73)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/datetime.h?r1=1.72&r2=1.73)