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>

arch/x86/include/asm/irq_vectors.h

@@ -34,11 +34,6 @@
  * (0x80 is the syscall vector, 0x30-0x3f are for ISA)
  */
 #define FIRST_EXTERNAL_VECTOR		0x20
-/*
- * We start allocating at 0x21 to spread out vectors evenly between
- * priority levels. (0x80 is the syscall vector)
- */
-#define VECTOR_OFFSET_START		1
 
 /*
  * Reserve the lowest usable vector (and hence lowest priority)  0x20 for