Remove hardcoded optimisation option.

Index: CMakeLists.txt
--- CMakeLists.txt.orig
+++ CMakeLists.txt
@@ -515,7 +515,6 @@ target_compile_options(freeorionparseobj
     PRIVATE
         $<$<CXX_COMPILER_ID:Clang>:-ftemplate-depth=512>
         $<$<CXX_COMPILER_ID:AppleClang>:-ftemplate-depth=512>
-        $<$<AND:$<NOT:$<BOOL:${BUILD_TESTING}>>,$<CXX_COMPILER_ID:GNU>>:-O3>
 )
 
 target_compile_definitions(freeorionparseobj
