meta-ibm: witherspoon: fans: drop unused variable

meta-phosphor dropped support for native class config recipes and uses
target class config recipes by default.  Drop the reference to
PHOSPHOR_FAN_CONFIG_USE_NATIVE_SYSROOT as it is no longer used in
meta-phosphor.

Change-Id: I5a259d7a63b70468f6301ca1f3f0487142d5d70c
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/meta-witherspoon/conf/machine/witherspoon.conf b/meta-witherspoon/conf/machine/witherspoon.conf
index c5af218..579b00d 100644
--- a/meta-witherspoon/conf/machine/witherspoon.conf
+++ b/meta-witherspoon/conf/machine/witherspoon.conf
@@ -2,7 +2,6 @@
 KERNEL_DEVICETREE = "${KMACHINE}-bmc-opp-${MACHINE}.dtb"
 KERNEL_FEATURES_append = " phosphor-wdt-on-panic"
 
-PHOSPHOR_FAN_CONFIG_USE_NATIVE_SYSROOT = "0"
 UBOOT_MACHINE = "ast_g5_ncsi_config"
 
 OBMC_POWER_SUPPLY_INSTANCES = "0 1"