build: remove always-true assertions

Summary:
Remove some always-true assertions.
They provoke these compilation failures:

  table/plain_table_key_coding.cc:279:20: error: comparison of unsigned expression >= 0 is always true [-Werror=type-limits]
  db/version_set.cc:336:15: error: comparison of unsigned expression >= 0 is always true [-Werror=type-limits]

* table/plain_table_key_coding.cc (rocksdb): Remove assertion that
unsigned type variable is >= 0.
* db/version_set.cc (DoGenerateLevelFilesBrief): Likewise.

Test Plan:
  Run "make EXTRA_CXXFLAGS='-W -Wextra'" and see fewer errors.

Reviewers: ljin, sdong, igor.sugak, igor

Reviewed By: igor

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D33747

db/version_set.cc

@@ -333,7 +333,6 @@ void DoGenerateLevelFilesBrief(LevelFilesBrief* file_level,
         const std::vector<FileMetaData*>& files,
         Arena* arena) {
   assert(file_level);
-  assert(files.size() >= 0);
   assert(arena);
 
   size_t num = files.size();

table/plain_table_key_coding.cc

@@ -276,7 +276,6 @@ Status PlainTableKeyDecoder::NextPrefixEncodingKey(
         if (seekable != nullptr) {
           *seekable = false;
         }
-        assert(prefix_len_ >= 0);
         cur_key_.Reserve(prefix_len_ + size);
 
         Slice tmp_slice;