[test] Remove unused 'test_examples' config var

[mgorny]

Summary

Remove config.test_examples from lit.site.cfg and the relevant
ENABLE_EXAMPLES definition from CMake. It is not used anywhere.

test/CMakeLists.txt

-if(LLVM_BUILD_EXAMPLES)
-  set(ENABLE_EXAMPLES 1)
-endif()
-
 configure_lit_site_cfg(
   ${CMAKE_CURRENT_SOURCE_DIR}/lit.site.cfg.in
   ${CMAKE_CURRENT_BINARY_DIR}/lit.site.cfg
   )
 configure_lit_site_cfg(
   ${CMAKE_CURRENT_SOURCE_DIR}/Unit/lit.site.cfg.in
   ${CMAKE_CURRENT_BINARY_DIR}/Unit/lit.site.cfg
   )

test/lit.site.cfg.in

 config.host_cxx = "@HOST_CXX@"
 config.host_ldflags = "@HOST_LDFLAGS@"
 config.llvm_use_intel_jitevents = "@LLVM_USE_INTEL_JITEVENTS@"
 config.llvm_use_sanitizer = "@LLVM_USE_SANITIZER@"
 config.have_zlib = "@HAVE_LIBZ@"
 config.have_libxar = "@HAVE_LIBXAR@"
 config.have_dia_sdk = @LLVM_ENABLE_DIA_SDK@
 config.enable_ffi = "@LLVM_ENABLE_FFI@"
-config.test_examples = "@ENABLE_EXAMPLES@"
 
 # Support substitution of the tools_dir with user parameters. This is
 # used when we can't determine the tool dir at configuration time.
 try:
     config.llvm_tools_dir = config.llvm_tools_dir % lit_config.params
     config.llvm_shlib_dir = config.llvm_shlib_dir % lit_config.params
 except KeyError:
     e = sys.exc_info()[1]
     key, = e.args
     lit_config.fatal("unable to find %r parameter, use '--param=%s=VALUE'" % (key,key))
 
 # Let the main config do the real work.
 lit_config.load_config(config, "@LLVM_SOURCE_DIR@/test/lit.cfg")