Remove the code of legacy VPN from Settings

IKEv2 VPN options are now functional, so removing the code of
legacy VPN. (i.e. PPTP, L2TP/IPSec PSK, L2TP/IPSec RSA,
IPSec Xauth PSK, IPSec Xauth RSA, IPSec Hybrid RSA)

Bug: 161776767
Test: Build pass, m
Test: Flash and manual test
Test: atest SettingsUnitTests:com.android.settings.vpn2
Change-Id: I7a017026adec2452254c78ef5e0ebf61ce1a9ecf