pgsql: Remove prepared transactions from main isolation test schedule.

Remove prepared transactions from main isolation test schedule.

There is no point in running this test when prepared transactions are disabled,
which is the default. New make targets that include the test are provided. This
will save some useless waste of cycles on buildfarm machines.

Backpatch to 9.1 where these tests were introduced.

Branch
------
REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/963bafde8b225a123e31649f44e26ed930b2002c

Modified Files
--------------
src/test/isolation/Makefile           |    7 +++++++
src/test/isolation/isolation_schedule |    1 -
2 files changed, 7 insertions(+), 1 deletions(-)