Deleted two unused assignments:

1. offset_in_block in Reader::SkipToInitialBlock().
2. in_fragmented_record in Reader::ReadRecord().

Reasons for the change:
1. offset_in_block is not read again after the if condition.
2. The kFullRecordType switch branch returns, so
   in_fragmented_record isn't read again.
3. The kFirstType switch branch sets in_fragmented_record to
   true after the if, so the write in the if is ignored.

Change contributed by @C0deAi on GitHub.

This fixes https://github.com/google/leveldb/issues/517

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=172763897