RelNotes: remove duplicate release note

In the 2.18 cycle, directory rename detection was merged, then reverted,
then reworked in such a way to fix another prominent bug in addition to
the original problem causing it to be reverted.  When the reworked series
was merged, we ended up with two nearly duplicate release notes.  Remove
the second copy, but preserve the information about the extra bug fix.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/RelNotes/2.18.0.txt

@@ -12,7 +12,9 @@ UI, Workflows & Features
    want to move to z/d by taking the hint that the entire directory
    'x' moved to 'z'.  A bug causing dirty files involved in a rename
    to be overwritten during merge has also been fixed as part of this
-   work.
+   work.  Incidentally, this also avoids updating a file in the
+   working tree after a (non-trivial) merge whose result matches what
+   our side originally had.
  * "git filter-branch" learned to use a different exit code to allow
    the callers to tell the case where there was no new commits to
@@ -256,16 +258,6 @@ Performance, Internal Implementation, Development Support etc.
    repository object (which in turn tells the API which object store
    the objects are to be located).
- * Rename detection logic in "diff" family that is used in "merge" has
-   learned to guess when all of x/a, x/b and x/c have moved to z/a,
-   z/b and z/c, it is likely that x/d added in the meantime would also
-   want to move to z/d by taking the hint that the entire directory
-   'x' moved to 'z'.  A bug causing dirty files involved in a rename
-   to be overwritten during merge has also been fixed as part of this
-   work.  Incidentally, this also avoids updating a file in the
-   working tree after a (non-trivial) merge whose result matches what
-   our side originally had.
-
  * "git pack-objects" needs to allocate tons of "struct object_entry"
    while doing its work, and shrinking its size helps the performance
    quite a bit.