remove SIGUSR1 escape; pkill <editor> is the standard recovery

The SIGUSR1 -> editor_emergency_kill wiring was a contrived escape
hatch. A console user with a stuck editor reaches for kill / pkill /
SIGKILL on the editor, not undocumented signals on the chat client.
editor_emergency_kill() stays for the internal Ctrl-Z abort path.

src/tools/editor.h

@@ -16,9 +16,7 @@
 
 gboolean launch_editor(gchar* initial_content, void (*callback)(gchar* content, void* data), void* user_data);
 
-// SIGUSR1 escape: SIGTERM the editor child. Use when WUNTRACED can't catch
-// it (alive but stuck). Wired in profanity.c.
-void editor_emergency_kill(void);
+void editor_emergency_kill(void); // SIGCONT+SIGTERM the editor child
 
 // TRUE if editor child is STOPPED (Ctrl-Z / :stop / gdb-attach). Does not
 // reap; g_child_watch keeps handling exit.