Just remove svn_fs_base__key_compare().

The function has different semantics than strcmp, but this doesn't matter for
how we use it.  Leaving the macro around essentially means we need to document
strcmp again which is silly.

* subversion/libsvn_fs_base/key-gen.h
  (svn_fs_base__key_compare): Remove.

* subversion/libsvn_fs_base/revs-txns.c
  (delete_txn_tree): Update callers to use strcmp directly.

* subversion/libsvn_fs_base/tree.c
  (get_copy_inheritance, txn_body_copied_from, txn_body_history_prev): ...


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1570753 13f79535-47bb-0310-9956-ffa450edef68