bootstrap: remove leftover empty variable dereference

This was missed in 9c0d1e7531edbebc7943d5b981353b17631ed632 when the for loop
that used this as loop variable was removed.

bootstrap

@@ -10,7 +10,7 @@ die() {
 cmake_extract_standard_flags()
 {
   sed -n "s/ *set *( *CMAKE_${2}${3}_EXTENSION_COMPILE_OPTION *\"\{0,1\}\([^\")]*\).*/\1/p" \
-    "${file}" "${cmake_source_dir}/Modules/Compiler/"${1:-*}-${2}.cmake 2>/dev/null | tr ';' ' '
+    "${cmake_source_dir}/Modules/Compiler/"${1:-*}-${2}.cmake 2>/dev/null | tr ';' ' '
 }
 
 # Version number extraction function.