Hexagon: Delete stale barrier.h

Commit 93ea02bb

 ("arch: Clean up asm/barrier.h implementations")
wired generic barrier.h for hexagon, but failed to delete the existing
file.

Cc: Richard Kuo <rkuo@codeaurora.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Compile-tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>