fsck doc: remove ancient out-of-date diagnostics

Remove diagnostics that haven't been emitted by "fsck" or its
predecessors for around 15 years. This documentation was added in
c64b9b8860 (Reference documentation for the core git commands.,
2005-05-05), but was out-of-date quickly after that.

Notes on individual diagnostics:

 - "expect dangling commits": Added in bcee6fd8e7 (Make 'fsck' able
   to[...], 2005-04-13), documented in c64b9b8860. Not emitted since
   1024932f01 (fsck-cache: walk the 'refs' directory[...],
   2005-05-18).

 - "missing sha1 directory": Added in 20222118ae (Add first cut at
   "fsck-cache"[...], 2005-04-08), documented in c64b9b8860. Not
   emitted since 230f13225d (Create object subdirectories on demand,
   2005-10-08).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-fsck.txt

@@ -129,14 +129,6 @@ using 'git commit-graph verify'. See linkgit:git-commit-graph[1].
 Extracted Diagnostics
 ---------------------
 
-expect dangling commits - potential heads - due to lack of head information::
-	You haven't specified any nodes as heads so it won't be
-	possible to differentiate between un-parented commits and
-	root nodes.
-
-missing sha1 directory '<dir>'::
-	The directory holding the sha1 objects is missing.
-
 unreachable <type> <object>::
 	The <type> object <object>, isn't actually referred to directly
 	or indirectly in any of the trees or commits seen. This can