delete_file() should force removal.

It's a shell command with a pty, but it's not really interactive,
so force the removal to avoid giving users dead-end prompts.

Bug: 17339227
Change-Id: Iaf5d95c49f032066aa741a711a2c45557d93c598
diff --git a/commandline.c b/commandline.c
index b268ca5..51c039e 100644
--- a/commandline.c
+++ b/commandline.c
@@ -1870,7 +1870,7 @@
     char buf[4096];
     char* quoted;
 
-    snprintf(buf, sizeof(buf), "shell:rm ");
+    snprintf(buf, sizeof(buf), "shell:rm -f ");
     quoted = escape_arg(filename);
     strncat(buf, quoted, sizeof(buf)-1);
     free(quoted);