Revert "Add vector of cleanup/termination events to be executed before quit"

This reverts commit 8c14f0f30f.

This list is not reliable - there are many ways for fish to quit that does not
invoke these functions. It's also not necessary since the history is correctly
saved on exec.

src/common.cpp

@@ -78,7 +78,6 @@ static void debug_shared(const wchar_t msg_level, const wcstring &msg);
 
 const wcstring whitespace = L" \t\r\n\v";
 const char *whitespace_narrow = " \t\r\n\v";
-std::stack<std::function<void()>> before_exit;
 
 bool is_whitespace(const wchar_t input) {
     for (auto c : whitespace) {