cmake: Remove useless call

Jan pointed out that it's useless to call this. If BUILD_TESTING is set,
testing is already enabled.

Change-Id: I37988ad9d8e9d13f3f5a1355ec864eea212fc7c2
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 67fa45e..7e70b41 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -139,7 +139,6 @@
 
 include(CTest)
 if(BUILD_TESTING)
-    enable_testing()
     find_package(trompeloeil 33 REQUIRED)
     find_package(doctest 2.3.1 REQUIRED)