Summary:
Upsteam PG commit 432bb9e04da4d4a1799b1fe7c723b975cb070c43 introduced JIT support. This commit
landed right just a few months before the pg11 release. Shortly before the release, the call was
made that JIT was not stable enough to be on by default in pg11, so it was disabled by commit
0fdadfb0192935e0f8bec7fa5ec9dbb470d6d587.

YB pg11 inherited the disabled JIT from pg11, and with the move to YB pg15, inherited enabled JIT
from pg15. Since we're not sure exactly how JIT interacts with YB pg15, disable it for now.

Also, D38011 / ddc084d1fd09c4cd8d82d4d47c937aee1070eeaf added `SET jit = false` lines in the index tests to get around the change of behaviour. Remove those lines.
Jira: DB-13392

Test Plan: Jenkins

Reviewers: tfoucher, fizaa

Reviewed By: tfoucher

Subscribers: yql

Differential Revision: https://phorge.dev.yugabyte.com/D39139