remove prefixscanrandom from db_bench help

Summary:
fix issue reported in https://github.com/facebook/rocksdb/issues/3757
Closes https://github.com/facebook/rocksdb/pull/3784

Differential Revision: D7794107

Pulled By: miasantreble

fbshipit-source-id: 43535074fcb82adb5656bcb916284b2dfc5cbb64

tools/db_bench_tool.cc

@@ -150,7 +150,6 @@ DEFINE_string(
     "reads\n"
     "\treadrandomwriterandom -- N threads doing random-read, "
     "random-write\n"
-    "\tprefixscanrandom      -- prefix scan N times in random order\n"
     "\tupdaterandom  -- N threads doing read-modify-write for random "
     "keys\n"
     "\txorupdaterandom  -- N threads doing read-XOR-write for "