gdbstub: drop gdbserver_cleanup in favour of gdb_exit

Despite it's name it didn't actually clean-up so let us document
gdb_exit() better and use that.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210108224256.2321-9-alex.bennee@linaro.org>
diff --git a/softmmu/runstate.c b/softmmu/runstate.c
index 636aab0..6177693 100644
--- a/softmmu/runstate.c
+++ b/softmmu/runstate.c
@@ -775,7 +775,7 @@
 
 void qemu_cleanup(void)
 {
-    gdbserver_cleanup();
+    gdb_exit(0);
 
     /*
      * cleaning up the migration object cancels any existing migration