shell-util: Remove shell_breakpoint

A near identical function appears in gjs, in the "system" module:

    const System = imports.system;
    System.breakpoint();

src/shell-util.c

@@ -678,18 +678,6 @@ shell_get_file_contents_utf8_sync (const char *path,
   return contents;
 }
 
-/**
- * shell_breakpoint:
- *
- * Using G_BREAKPOINT(), interrupt the current process.  This is useful
- * in conjunction with a debugger such as gdb.
- */
-void
-shell_breakpoint (void)
-{
-  G_BREAKPOINT ();
-}
-
 /**
  * shell_parse_search_provider:
  * @data: description of provider