[Cleanup] Redundant substring() operation in help.base

Removes the second substring() parameter if this parameter is the length
of the string. It's the default value.

Found during testing of Bug 569800 (new JDT cleanup for this case).

Change-Id: Ide197063514f401e233837fd696717814ade3cec
Signed-off-by: Lars Vogel <Lars.Vogel@vogella.com>