oe-find-native-sysroot: don't clear OECORE_NATIVE_SYSROOT

The OECORE_NATIVE_SYSROOT may come from environment vars, so don't clear
it, otherwise it broke runqemu-extract-sdk on sdk.

[YOCTO #11409]

(From OE-Core rev: 91faa9132821db4e7010b79943b2cfb804d6f4c5)

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/oe-find-native-sysroot

@@ -47,7 +47,6 @@ fi
 
 # Global vars
 BITBAKE_E=""
-OECORE_NATIVE_SYSROOT=""
 set_oe_native_sysroot(){
     echo "Running bitbake -e $1"
     BITBAKE_E="`bitbake -e $1`"