rules.mk: remove redundant target flags

rules.mk: remove redundant target flags

We're patching the GCC specs [1], [2] to implicitly add
$STAGING_DIR/usr/lib to the linker and $STAGING_DIR/usr/include to the
CPP flags. There is no need to globally pass these as -I and -L flags
respectively.

1. https://git.openwrt.org/?p=openwrt/openwrt.git;a=blob;f=toolchain/gcc/final/Makefile#l86
2. https://git.openwrt.org/?p=openwrt/openwrt.git;a=blob;f=scripts/patch-specs.sh#l37

Ref: https://patchwork.ozlabs.org/project/openwrt/patch/20200820060637.533293-1-a.heider@gmail.com/#2511505
Suggested-by: Jo-Philipp Wich <jo@mein.io>
Signed-off-by: Petr Štetiar <ynezz@true.cz>