arm64: Remove duplicate ttbr0 field in thread_info

A bad merge resolution resulted in two copies of the
ttbr0 field in the arm64 thread_info structure, causing
builds with CONFIG_ARM64_SW_TTBR0_PAN to fail. Eliminate
one.

Change-Id: I14689d3d2c8a1804f16530874336a0e989af8643
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>