DfsInserter: Read minBytesForObjectSizeIndex only from repo config

DfsInserter: Read minBytesForObjectSizeIndex only from repo config

In general, JGit reads the configuration it needs from the repository
configuration. minBytesForObjectSizeIndex is a special case with a
setter for subclasses but that is unnecessary.

Remove the setter and read the conf from the repo. Make the property
final and read it directly from the conf (it is clearer than parsing a
whole PackConfig to read a single value).

Change-Id: I8d77247452ff65e6c431fdcfebb90ed2ce40aed1