arm64: remove redundant header file in current.h

Commint 9d84fb27 ("arm64: restore get_current() optimisation") has
removed read_sysreg() and asm/sysreg.h is redundant.

This patch removes asm/sysreg.h header file.

Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Shaokun Zhang <zhangshaokun@hisilicon.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>