Fix build with Boost 1.91
Remove hardcoded optimisation option.

Index: CMakeLists.txt
--- CMakeLists.txt.orig
+++ CMakeLists.txt
@@ -283,7 +283,6 @@ find_package(Boost ${MINIMUM_BOOST_VERSION}
         log
         log_setup
         serialization
-        system
         thread
     REQUIRED)
 
@@ -515,7 +514,6 @@ 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
