db_bench: not need to check mmap for PlainTable

Summary:
PlainTable now supports non-mmap mode. We don't need to check it anymore.
Closes https://github.com/facebook/rocksdb/pull/1882

Differential Revision: D4751643

Pulled By: siying

fbshipit-source-id: ab14540

tools/db_bench_tool.cc

@@ -2851,10 +2851,6 @@ class Benchmark {
           FLAGS_rep_factory != kHashLinkedList) {
         fprintf(stderr, "Waring: plain table is used with skipList\n");
       }
-      if (!FLAGS_mmap_read && !FLAGS_mmap_write) {
-        fprintf(stderr, "plain table format requires mmap to operate\n");
-        exit(1);
-      }
 
       int bloom_bits_per_key = FLAGS_bloom_bits;
       if (bloom_bits_per_key < 0) {