Clean up debug output

Remove the unnecessary util.c/util.h files and use purple_debug_info()
throughout instead. This is more direct. Also replace all usages of
printf() with the same helper.

This makes the Delta plugin usable in command-line applications like
Finch. Previously, the printf() output was messing up the screen!

libdelta.h

@@ -27,4 +27,6 @@
 #define PLUGIN_ACCOUNT_OPT_SMTP_USER        "send_user"
 #define PLUGIN_ACCOUNT_OPT_SMTP_PASS        "send_pw"
 
+#define UNUSED(x) (void)(x)
+
 #endif