msm: restart: Remove MSM_RESTART_V2 distinction

All platforms set the Kconfig symbol MSM_RESTART_V2, so we can just remove
the symbol and associated code for the non-v2 support.

Change-Id: I93dcb5e79828aef88c9b1f20a170cb255cc095c1
Signed-off-by: Kumar Gala <galak@codeaurora.org>