build.config: drop setting of BUILDTOOLS_PREBUILT_BIN

to use the default value specified in build.config.common.
Otherwise the tools in build-tools used will not be found,
like the build error reported here[1]:
    build/build.sh: line 983: soong_zip: command not found

[1]: https://ci.android.com/builds/submitted/9574600/kernel/latest/view/logs/build_error.log

Test: build the x15 4.14 kernel successfully

Change-Id: I3a40d42ecb37dcbff527291eff7796162f62f3e9
Signed-off-by: Yongqin Liu <yongqin.liu@linaro.org>
diff --git a/build.config b/build.config
index 826f603..a7ca6a0 100644
--- a/build.config
+++ b/build.config
@@ -14,7 +14,6 @@
 EXTRA_CMDS=''
 LINUX_GCC_CROSS_COMPILE_PREBUILTS_BIN=prebuilts/gcc/linux-x86/arm/arm-linux-androideabi-4.9/bin
 
-BUILDTOOLS_PREBUILT_BIN=''
 IN_KERNEL_MODULES=1
 DO_NOT_STRIP_MODULES=1