Cleanup dead code

This patch removes some dead code in exec.c
Signed-off-by: NJun Koi <junkoi2004@gmail.com>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

exec.c

@@ -2884,15 +2884,12 @@ void *qemu_get_ram_ptr(ram_addr_t addr)
    (typically a TLB entry) back to a ram offset.  */
 ram_addr_t qemu_ram_addr_from_host(void *ptr)
 {
-    RAMBlock *prev;
     RAMBlock *block;
     uint8_t *host = ptr;
 
-    prev = NULL;
     block = ram_blocks;
     while (block && (block->host > host
                      || block->host + block->length <= host)) {
-        prev = block;
         block = block->next;
     }
     if (!block) {