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/postfork.cpp

@@ -109,9 +109,9 @@ bool set_child_group(job_t *j, process_t *p, int print_errors) {
         int result = -1;
         errno = EINTR;
         while (result == -1 && errno == EINTR) {
-            signal_block(true);
+            signal_block();
             result = tcsetpgrp(STDIN_FILENO, j->pgid);
-            signal_unblock(true);
+            signal_unblock();
         }
         if (result == -1) {
             if (errno == ENOTTY) redirect_tty_output();
@@ -251,8 +251,6 @@ int setup_child_process(job_t *j, process_t *p, const io_chain_t &io_chain) {
         signal_reset_handlers();
     }
 
-    signal_unblock();  // remove all signal blocks
-
     return ok ? 0 : -1;
 }