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_2_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/a8f0f98f9b9332f6dcd5bcbc0491de4410868622

Modified Files
--------------
src/test/isolation/Makefile           |   10 ++++++++++
src/test/isolation/isolation_schedule |    1 -
2 files changed, 10 insertions(+), 1 deletions(-)