Remove 'autoinit' DB setting

It doesn't make sense to do a load->database() call
but not connect to the database. IIRC there was more
stuff in CI_DB_driver::initialize() at some point,
so that was probably the reason why the setting
existed in the first place. However, now it only
results in users making invalid bug reports because
they don't understand the feature ...

Examples during just the past 2 weeks: #3571 #3601 #3607
diff --git a/user_guide_src/source/changelog.rst b/user_guide_src/source/changelog.rst
index a904c82..8f77f36 100644
--- a/user_guide_src/source/changelog.rst
+++ b/user_guide_src/source/changelog.rst
@@ -155,6 +155,8 @@
    -  DEPRECATED the 'mysql', 'sqlite', 'mssql' and 'pdo/dblib' (also known as 'pdo/mssql' or 'pdo/sybase') drivers.
    -  Added **dsn** configuration setting for drivers that support DSN strings (PDO, PostgreSQL, Oracle, ODBC, CUBRID).
    -  Added **schema** configuration setting (defaults to *public*) for drivers that might need it (currently used by PostgreSQL and ODBC).
+   -  Added **save_queries** configuration setting to *application/config/database.php* (defaults to ``TRUE``).
+   -  Removed **autoinit** configuration setting as it doesn't make sense to instantiate the database class but not connect to the database.
    -  Added subdrivers support (currently only used by PDO).
    -  Added an optional database name parameter to ``db_select()``.
    -  Removed ``protect_identifiers()`` and renamed internal method ``_protect_identifiers()`` to it instead - it was just an alias.
@@ -173,7 +175,6 @@
    -  Added support for SQLite3 database driver.
    -  Added Interbase/Firebird database support via the *ibase* driver.
    -  Added ODBC support for ``create_database()``, ``drop_database()`` and ``drop_table()`` in :doc:`Database Forge <database/forge>`.
-   -  Added **save_queries** configuration setting to *application/config/database.php* (defaults to ``TRUE``).
    -  Added support to binding arrays as ``IN()`` sets in ``query()``.
 
    -  :doc:`Query Builder <database/query_builder>` changes include: