[libc++] Remove test that is irrelevant since f1a96de

We don't want to give the impression that we allow running arbitrary
shell commands under %{exec}, which isn't the case.

libcxx/test/libcxx/selftest/shell-escape.sh.cpp → libcxx/test/libcxx/selftest/shell-no-escape-builtins.sh.cpp

@@ -6,13 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-// Make sure that arguments of the %{exec} substitution are shell-escaped
-// properly. If that wasn't the case, the command would fail because the
-// shell would look for a matching `"`.
-
-// RUN: %{exec} echo '"'
-
-// Also make sure that we don't escape Shell builtins like `!`, because the
+// Make sure that we don't escape Shell builtins like `!`, because the
 // shell otherwise thinks it's a command and it can't find it.
 
 // RUN: ! false