dat: one pair of content purposes

Two frontends read the same two numbers, and wl.c carried four lines of
comment to say why it kept its own copy of them.  IBus and text-input-v3
number the purposes alike because both took them from GTK, so the header
is where they belong and the explanation goes away with the copy.  hidden
stays in both files: one asks about a context, the other about the seat.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

wl.c

@@ -9,16 +9,9 @@
 #include "imv2.h"
 #include "vkv1.h"
 
-/*
- * text-input-v3 content purposes whose keys the engine never sees.  IBus
- * numbers these alike but does not share them: one enum here is cheaper
- * than reaching into dat.h and ibus.c both.
- */
 enum
 {
-	Purposepassword	= 8,
-	Purposepin	= 9,
-	Maxcode		= 0x300,	/* KEY_MAX+1: what evdev can send */
+	Maxcode	= 0x300,	/* KEY_MAX+1: what evdev can send */
 };
 
 /*