arm64: barrier: Remove spec_bar() macro

The spec_bar() macro was introduced in
commit bd4fb6d2 ("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 3d2403fd

 ("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>