[BUGFIX] Update enableDeprecationLog default value and help text

TYPO3's deprecation_log is disabled by default. The Install Tool
shows an incorrect help text for the configuration
$GLOBALS['TYPO3_CONF_VARS']['SYS']['enableDeprecationLog'], claiming
that the default value is "file". In addition, enableDeprecationLog
is set to "file", which should be empty.

The patch removes the value of enableDeprecationLog and removes the
statement about the default value: other values in this area do not
state the default value either and since TYPO3 CMS 6.2 LTS, the
default value depends on the Application Context.

Resolves: #53653
Releases: 6.2
Change-Id: I2061a66b03e1c194f86d346e4183e95d3e959922
Reviewed-on: https://review.typo3.org/25417
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn