gcc: drop outdated sed for system headers clobber

When reviewing NixOS#181802 (comment)
I noticed outdated code that attempted to override /usr/include.

    sed -i \
        -e "s,glibc_header_dir=/usr/include,glibc_header_dir=$libc_dev/include", \
        gcc/configure

`glibc_header_dir` was removed from `gcc-4.6` and later in
https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=6961669f48aa18168b2d7daa7e2235fbec7cb636
(Dec 2010, "(gcc_cv_ld_eh_frame_hdr): Only check GNU ld for  --eh-frame-hdr.").

Since then gcc got `--with-native-system-header-dir=` which `nixpkgs` uses
for all packaged `gcc` versions.

The change should be a no-op.

pkgs/development/compilers/gcc/builder.sh

@@ -171,15 +171,6 @@ preConfigure() {
         rm -Rf zlib
     fi
 
-    if test -f "$NIX_CC/nix-support/orig-libc"; then
-        # Patch the configure script so it finds glibc headers.  It's
-        # important for example in order not to get libssp built,
-        # because its functionality is in glibc already.
-        sed -i \
-            -e "s,glibc_header_dir=/usr/include,glibc_header_dir=$libc_dev/include", \
-            gcc/configure
-    fi
-
     if test -n "$crossMingw" -a -n "$crossStageStatic"; then
         mkdir -p ../mingw
         # --with-build-sysroot expects that: