stdenv-setup: Remove superfluous check for /bin/sh

The line was essentially checking whether /bin/sh exists and is
executable and if that's the case, the isScript function returns
successfully.

When asking the author of this line on IRC it seems that even they can't
remember or imagine what this was supposed to be.

In summary: Whenever /bin/sh doesn't exist during a build, *any* file
given to isScript is reported as being a script even if it isn't.

This is kinda counter-intuitive and not something what somebody would
expect from a function called "isScript".

Signed-off-by: aszlig <aszlig@nix.build>
Cc: @edolstra

pkgs/stdenv/generic/setup.sh

@@ -220,7 +220,6 @@ isScript() {
     local fn="$1"
     local fd
     local magic
-    if ! [ -x /bin/sh ]; then return 0; fi
     exec {fd}< "$fn"
     read -r -n 2 -u "$fd" magic
     exec {fd}<&-