Remove GD32 platform entries from database

Removed obsolete platform entries for GD32 series.

Author: JohnK1987

tools/python/mbed_os_tools/detect/platform_database.py

@@ -255,9 +255,6 @@
         u"1549": u"LPC1549",
         u"1600": u"LPC4330_M4",
         u"1605": u"LPC4330_M4",
-        u"1701": u"GD32_F307VG",
-        u"1702": u"GD32_F450ZI",
-        u"1703": u"GD32_E103VB",
         u'1900': u'CY8CKIT_062_WIFI_BT',
         u'1901': u'CY8CPROTO_062_4343W',
         u'1902': u'CY8CKIT_062_BLE',