Drop the converters for old data

2.15.0 stored all settings, schedules and pages on the display in the
current formats (old_data_left 0), so only those are read now: settings
version 7, schedule version 4, page format 7. The one-time migrations
(icons from 2.10.0, data from 2.15.0) are removed too. About 9 KB less
flash. CLAUDE.md says how to keep data over a future format change.

Version 2.15.1.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/esp32/config.h

@@ -118,7 +118,7 @@ const uint32_t SETUP_AP_DELAY = 20000;        // open the AP when WiFi fails thi
 const uint32_t WIFI_CONNECT_TIMEOUT = 30000;
 const uint32_t WIFI_RESTART_TIME = 10UL * 60 * 1000;
 
-#define FIRMWARE_VERSION "2.15.0"
+#define FIRMWARE_VERSION "2.15.1"
 
 // Home Assistant device id (do not change: Home Assistant's entities are linked to it)
 const uint8_t HA_DEVICE_ID[] = {0x34, 0x86, 0x5d, 0x3a, 0x08, 0x28};