Remove unused `constexpr strlen` check

Summary:
Superseded by `<folly/portability/Constexpr.h>`'s `constexpr_strlen()`.
Closes https://github.com/facebook/folly/pull/585

Reviewed By: yfeldblum

Differential Revision: D4973136

Pulled By: Orvid

fbshipit-source-id: ada9bedf53ce219b0924c3f7aaad75bce9605f0f

folly/configure.ac

@@ -251,21 +251,6 @@ AC_DEFINE_UNQUOTED(
   [OVERRIDE], [$override_val],
   [Define to "override" if the compiler supports C++11 "override"])
 
-AC_CACHE_CHECK(
-  [for constexpr strlen],
-  [folly_cv_func_constexpr_strlen],
-  [AC_COMPILE_IFELSE(
-    [AC_LANG_SOURCE[
-      #include <cstring>
-      static constexpr int val = strlen("foo");]],
-    [folly_cv_func_constexpr_strlen=yes],
-    [folly_cv_func_constexpr_strlen=no])])
-
-if test "$folly_cv_func_constexpr_strlen" = yes; then
-    AC_DEFINE([HAVE_CONSTEXPR_STRLEN], [1],
-              [Define to 1 if strlen(3) is constexpr.])
-fi
-
 AC_CACHE_CHECK(
   [for libc++],
   [folly_cv_lib_libcpp],