arm64: barrier: Remove spec_bar() macro

The spec_bar() macro was introduced in
commit bd4fb6d270 ("arm64: Add support for SB barrier and patch in over DSB; ISB sequences")
as a way for C to insert a speculation barrier and was then
used in one single place: set_fs().

Later on
commit 3d2403fd10 ("arm64: uaccess: remove set_fs()")
deleted set_fs() altogether and as noted in the commit
on the new path the regular sb() assembly macro will
be used.

Delete the remnant.

Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20210325141304.1607595-1-linus.walleij@linaro.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/barrier.h

@@ -25,10 +25,6 @@
 #define psb_csync()	asm volatile("hint #17" : : : "memory")
 #define csdb()		asm volatile("hint #20" : : : "memory")
 
-#define spec_bar()	asm volatile(ALTERNATIVE("dsb nsh\nisb\n",		\
-						 SB_BARRIER_INSN"nop\n",	\
-						 ARM64_HAS_SB))
-
 #ifdef CONFIG_ARM64_PSEUDO_NMI
 #define pmr_sync()						\
 	do {							\