finish cleanup of signal blocking code

PR #3691 made most calls to `signal_block()` and `signal_unblock()`
no-ops unless a magic env var is set when fish starts running. It's
been seven months since that change was made and no problems have been
reported. This finishes that work by removing those no-op function calls
and support for the magic env var in our next major release (which won't
happen till at least six months from now).

src/reader.cpp

@@ -1575,20 +1575,12 @@ static void reader_interactive_init() {
     // Check if we are in control of the terminal, so that we don't do semi-expensive things like
     // reset signal handlers unless we really have to, which we often don't.
     if (tcgetpgrp(STDIN_FILENO) != shell_pgid) {
-        int block_count = 0;
-        int i;
-
         // Bummer, we are not in control of the terminal. Stop until parent has given us control of
-        // it. Stopping in fish is a bit of a challange, what with all the signal fidgeting, we need
-        // to reset a bunch of signal state, making this coda a but unobvious.
+        // it.
         //
         // In theory, reseting signal handlers could cause us to miss signal deliveries. In
-        // practice, this code should only be run suring startup, when we're not waiting for any
+        // practice, this code should only be run during startup, when we're not waiting for any
         // signals.
-        while (signal_is_blocked()) {
-            signal_unblock();
-            block_count++;
-        }
         signal_reset_handlers();
 
         // Ok, signal handlers are taken out of the picture. Stop ourself in a loop until we are in
@@ -1632,10 +1624,6 @@ static void reader_interactive_init() {
         }
 
         signal_set_handlers();
-
-        for (i = 0; i < block_count; i++) {
-            signal_block();
-        }
     }
 
     // Put ourselves in our own process group.