postfix: remove overriding config config_directory

postfix 2.11.1 adds overriding config check. In postinstall script, it
calls command newaliases to read configure file main.cf_2.0.
config_directory is set in config main.cf_2.0, but it is an overriding
config and warning shows when build an image:

WARNING: log_check: There is a warn message in the logfile
WARNING: log_check: Matched keyword: [warn]
WARNING: log_check: newaliases: warning:
/buildarea3/kkang/poky/qemuarm-build/tmp/work/qemuarm-poky-linux-gnueabi/core-
image-minimal/1.0-r0/rootfs/etc/postfix/main.cf, line 27: overriding
earlier entry: config_directory=/buildarea3/kkang/
poky/qemuarm-build/tmp/work/qemuarm-poky-linux-gnueabi/core-image-minimal/1.0-r0/rootfs/etc/postfix

Remove config_directory setting in main.cf_2.0 to avoid this warning.

Signed-off-by: Kai Kang <kai.kang@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>