x86/vector: Remove the macro VECTOR_OFFSET_START

x86/vector: Remove the macro VECTOR_OFFSET_START

Now, Linux uses matrix allocator for vector assignment, the original
assignment code which used VECTOR_OFFSET_START has been removed.

So remove the stale macro as well.

Fixes: commit 69cde0004a4b ("x86/vector: Use matrix allocator for vector assignment")
Signed-off-by: Dou Liyang <douly.fnst@cn.fujitsu.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: David Rientjes <rientjes@google.com>
Cc: hpa@zytor.com
Link: https://lkml.kernel.org/r/20180425020553.17210-1-douly.fnst@cn.fujitsu.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>