Remove _DARWIN_C_SOURCE definition for bundled sources on macOS

Reverts a change introduced in ab44b0c. This was previously in mongodb/mongo-c-driver#920 but removed before merging in mongodb/mongo-c-driver@9d2d8b1.

Author: jmikola

scripts/autotools/libbson/FindDependencies.m4

@@ -2,11 +2,6 @@ if test "$os_win32" != "yes"; then
     PHP_MONGODB_BUNDLED_CFLAGS="$PHP_MONGODB_BUNDLED_CFLAGS -D_DEFAULT_SOURCE"
 fi
 
-# Enable macOS extensions for strlcpy and arc4random
-if test "$os_darwin" = "yes"; then
-    PHP_MONGODB_BUNDLED_CFLAGS="$PHP_MONGODB_BUNDLED_CFLAGS -D_DARWIN_C_SOURCE=1"
-fi
-
 # Check for strnlen()
 dnl AC_CHECK_FUNC isn't properly respecting _XOPEN_SOURCE for strnlen for unknown reason
 AC_SUBST(BSON_HAVE_STRNLEN, 0)