arm64: kernel: compiling issue, need delete read_current_timer()

Under arm64, we will calibrate the delay loop statically using a known
timer frequency, so delete read_current_timer(), or it will cause
compiling issue with allmodconfig.

The related error:
  ERROR: "read_current_timer" [lib/rbtree_test.ko] undefined!
  ERROR: "read_current_timer" [lib/interval_tree_test.ko] undefined!
  ERROR: "read_current_timer" [fs/ext4/ext4.ko] undefined!
  ERROR: "read_current_timer" [crypto/tcrypt.ko] undefined!

Signed-off-by: Chen Gang <gang.chen@asianux.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Git-commit: 6916b14ea140ff5c915895eefe9431888a39a84d
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Ian Maund <imaund@codeaurora.org>