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[email protected]/#2511505
Suggested-by: Jo-Philipp Wich <[email protected]>
Signed-off-by: Petr Štetiar <[email protected]>