Revert "finish cleanup of signal blocking code"

This reverts commit fb08fe5f47.

Needed to cleanly apply PR#4268. Will reapply after applying that
change.

src/fish.cpp

@@ -387,6 +387,13 @@ int main(int argc, char **argv) {
 
     const io_chain_t empty_ios;
     if (read_init(paths)) {
+        // TODO: Remove this once we're confident that not blocking/unblocking every signal around
+        // some critical sections is no longer necessary.
+        env_var_t fish_no_signal_block = env_get_string(L"FISH_NO_SIGNAL_BLOCK");
+        if (!fish_no_signal_block.missing_or_empty() && !from_string<bool>(fish_no_signal_block)) {
+            ignore_signal_block = false;
+        }
+
         // Stomp the exit status of any initialization commands (issue #635).
         proc_set_last_status(STATUS_CMD_OK);