arm64: barrier: Remove spec_bar() macro

arm64: barrier: Remove spec_bar() macro

The spec_bar() macro was introduced in
commit bd4fb6d270bc ("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 3d2403fd10a1 ("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 <[email protected]>
Signed-off-by: Linus Walleij <[email protected]>
Acked-by: Mark Rutland <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Catalin Marinas <[email protected]>