Don't specify CMAKE_CXX_STANDARD anymore either.

This is for consistency with commit a022cc0.

Change-Id: Iaded0d669d821b06bdb57d70b519205426643f91
Reviewed-on: https://code-review.googlesource.com/c/re2/+/59870
Reviewed-by: Perry Lorier <perryl@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 62d4995..e10de15 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -10,11 +10,6 @@
 include(CTest)
 include(GNUInstallDirs)
 
-if(NOT CMAKE_CXX_STANDARD)
-  set(CMAKE_CXX_STANDARD 11)
-  set(CMAKE_CXX_STANDARD_REQUIRED ON)
-endif()
-
 option(BUILD_SHARED_LIBS "build shared libraries" OFF)
 option(USEPCRE "use PCRE in tests and benchmarks" OFF)