stdenv: remove isMachO helper function

This reverts commit 488395c0f8.

Currently, `nix print-dev-env` fails to execute if this function is present, because of its use of hex literals.
Until this issue (https://github.com/NixOS/nix/issues/5262) is solved, we should revert this to prevent breakage.

pkgs/stdenv/generic/setup.sh

@@ -199,29 +199,6 @@ isELF() {
     if [ "$magic" = $'\177ELF' ]; then return 0; else return 1; fi
 }
 
-# Return success if the specified file is a Mach-O object.
-isMachO() {
-    local fn="$1"
-    local fd
-    local magic
-    exec {fd}< "$fn"
-    read -r -n 4 -u "$fd" magic
-    exec {fd}<&-
-    # https://opensource.apple.com/source/lldb/lldb-310.2.36/examples/python/mach_o.py.auto.html
-    if [[ "$magic" = $'\xfe\xed\xfa\xcf' || "$magic" = $'\xcf\xfa\xed\xfe' ]]; then
-        # MH_MAGIC_64 || MH_CIGAM_64
-        return 0;
-    elif [[ "$magic" = $'\xfe\xed\xfa\xce' || "$magic" = $'\xce\xfa\xed\xfe' ]]; then
-        # MH_MAGIC || MH_CIGAM
-        return 0;
-    elif [[ "$magic" = $'\xca\xfe\xba\xbe' || "$magic" = $'\xbe\xba\xfe\xca' ]]; then
-        # FAT_MAGIC || FAT_CIGAM
-        return 0;
-    else
-        return 1;
-    fi
-}
-
 # Return success if the specified file is a script (i.e. starts with
 # "#!").
 isScript() {