fsck doc: remove ancient out-of-date diagnostics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
c64b9b8 (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2005-05-05), but was out-of-date quickly after that.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 bcee6fd (Make 'fsck' able
   to[...], 2005-04-13), documented in c64b9b8. Not emitted since
   1024932 (fsck-cache: walk the 'refs' directory[...],
   2005-05-18).

 - "missing sha1 directory": Added in 2022211 (Add first cut at
   "fsck-cache"[...], 2005-04-08), documented in c64b9b8. Not
   emitted since 230f132 (Create object subdirectories on demand,
   2005-10-08).

Signed-off-by: Ævar Arnfjörð Bjarmason <[email protected]>
Reviewed-by: Taylor Blau <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: avar

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