classes/buildhistory: remove default of BUILDHISTORY_CHECKVERBACKWARDS

Since this variable is no longer used, we don't need to set a default
for it.

(From OE-Core rev: a5230835c539781b5b035dc6d0be3cac5a5bd305)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/buildhistory.bbclass

@@ -16,7 +16,6 @@ BUILDHISTORY_IMAGE_FILES ?= "/etc/passwd /etc/group"
 BUILDHISTORY_COMMIT ?= "0"
 BUILDHISTORY_COMMIT_AUTHOR ?= "buildhistory <buildhistory@${DISTRO}>"
 BUILDHISTORY_PUSH_REPO ?= ""
-BUILDHISTORY_CHECKVERBACKWARDS ?= "1"
 
 # Must inherit package first before changing PACKAGEFUNCS
 inherit package