[TSan] Cleanup TSan runtime support for Go on linux-aarch64. NFC.

This is a follow-up patch to r342541. After further investigations, only
48bits VMA size can be supported. As this is enforced in function
InitializePlatformEarly from lib/rt1/tsan_platform_linux.cc, the access
to the global variable vmaSize variable + switch can be removed. This
also addresses a comment from https://reviews.llvm.org/D52167.

vmaSize of 39 or 42bits are not compatible with a Go program memory
layout as the Go heap will not fit in the shadow memory area.

Patch by: Fangming Fang <Fangming.Fang@arm.com>

llvm-svn: 344329