Remove dir configuration in redis-3.0.conf

[mistoll]

• The value is overwritten in local conf anyway

Fixes this issue:

[ WARN] Hub : redis-server not found
[ WARN] Hub :     either you can't look up the admin PATH (ok)
[ WARN] Hub :     or it is not installed - hint 'rosdep install rocon_hub'
[INFO] [1545991000.959245]: Hub : version 3.0.6
7137:C 28 Dec 09:56:42.826 # Can't chdir to '/var/lib/redis': Permission denied
[FATAL] Hub : could not connect to the redis server - is it running?

Using redis-server 2:3.0.6-1ubuntu0.3 (armhf)