stdenvBootstrapTools: drop no-longer-needed, now-broken cross clause

The comment stating that "GCC has certain things built in
statically" is no longer true as of
#209870.  Moreover, as pointed
out in #228130 this
optionalString block no longer builds for
pkgsCross.*.freshBootstrapTools.

It is safe to drop this clause now.

pkgs/stdenv/linux/make-bootstrap-tools.nix

@@ -158,12 +158,6 @@ in with pkgs; rec {
         cp -d ${libmpc.out}/lib/libmpc*.so* $out/lib
         cp -d ${zlib.out}/lib/libz.so* $out/lib
 
-      '' + lib.optionalString (stdenv.hostPlatform != stdenv.buildPlatform) ''
-        # These needed for cross but not native tools because the stdenv
-        # GCC has certain things built in statically. See
-        # pkgs/stdenv/linux/default.nix for the details.
-        cp -d ${isl_0_20.out}/lib/libisl*.so* $out/lib
-
       '' + lib.optionalString (stdenv.hostPlatform.isRiscV) ''
         # libatomic is required on RiscV platform for C/C++ atomics and pthread
         # even though they may be translated into native instructions.