[libunwind] Remove unused substitution in AIX libunwind config

It must have been a copy-paste error, since cxx-include is never defined
by the libunwind config.

Differential Revision: https://reviews.llvm.org/D131030

libunwind/test/configs/ibm-libunwind-shared.cfg.in

@@ -5,7 +5,7 @@ lit_config.load_config(config, '@CMAKE_CURRENT_BINARY_DIR@/cmake-bridge.cfg')
 
 config.substitutions.append(('%{flags}', ''))
 config.substitutions.append(('%{compile_flags}',
-    '-nostdinc++ -I %{include} -I %{cxx-include}'
+    '-nostdinc++ -I %{include}'
 ))
 config.substitutions.append(('%{link_flags}',
     '-nostdlib++ -L %{lib} -lunwind -ldl -Wl,-bbigtoc'