Toolchain: Don't pre-build LibC and LibM, nor pre-install their headers

We can do away with that shenanigans now that libstdc++ is gone.
Also, simplify the toolchain dependency hash calculation to only depend
on the toolchain build script(s) and the Patches files we use to modify
the toolchain itself.

Toolchain/BuildIt.sh

@@ -225,20 +225,6 @@ pushd "$DIR/Build/"
         echo "XXX install gcc and libgcc"
         "$MAKE" install-gcc install-target-libgcc || exit 1
 
-        echo "XXX serenity libc and libm"
-        mkdir -p "$BUILD"
-        pushd "$BUILD"
-            cmake ..
-            "$MAKE" LibC
-            install -D Libraries/LibC/libc.a Libraries/LibM/libm.a Libraries/LibCxx/libstdc++.a Root/usr/lib/
-            SRC_ROOT=$(realpath "$DIR"/..)
-            for header in "$SRC_ROOT"/Libraries/Lib{C,M}/**/*.h; do
-                target=$(echo "$header" | sed -e "s@$SRC_ROOT/Libraries/LibC@@" -e "s@$SRC_ROOT/Libraries/LibM@@")
-                install -D "$header" "Root/usr/include/$target"
-            done
-            unset SRC_ROOT
-        popd
-
         if [ "$(uname -s)" = "OpenBSD" ]; then
             cd "$DIR/Local/libexec/gcc/i686-pc-serenity/$GCC_VERSION" && ln -sf liblto_plugin.so.0.0 liblto_plugin.so
         fi