Remove preferred kernel version from ast2400 machine

This was left over from when the ast2400 systems used a different kernel
than the ast2500 systems. Now that they are both on 4.6, we can drop
this PREFERRED_VERSION.

Change-Id: I683fdfdc804789fe1353ef09154c92714009cd07
Signed-off-by: Joel Stanley <joel@jms.id.au>
diff --git a/meta-ast2400/conf/machine/include/ast2400.inc b/meta-ast2400/conf/machine/include/ast2400.inc
index d67959a..a495f20 100644
--- a/meta-ast2400/conf/machine/include/ast2400.inc
+++ b/meta-ast2400/conf/machine/include/ast2400.inc
@@ -3,7 +3,6 @@
 #@DESCRIPTION: Common machine configuration for the Aspeed AST2400 Chip
 
 PREFERRED_PROVIDER_virtual/kernel ?= "linux-obmc"
-PREFERRED_VERSION_linux-obmc ?= "4.4+git%"
 PREFERRED_VERSION_u-boot ?= "v2016.05+git%"
 PREFERRED_VERSION_u-boot-fw-utils ?= "v2016.05+git%"