There is no DATA directory in BeOS, and its FindDir() doesn't implement
it. No need for a confusing backwards compatibility to something that
doesn't exist (had my hopes up, was going to move some non-executable
files from AddOns to Data in a program that works in BeOS and Haiku).
The removed enum label doesn't change the directory_which enum order
or count, as it was aliasing the value of another existing enum label
(B_SYSTEM_DATA_DIRECTORY).
Fixes#13470
Signed-off-by: Axel Dörfler <axeld@pinc-software.de>