Revert "core.abbrevguard: Ensure short object names stay unique a bit longer"

This reverts commit 72a5b561fc, as adding
fixed number of hexdigits more than necessary to make one object name
locally unique does not help in futureproofing the uniqueness of names
we generate today.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

environment.c

@@ -21,7 +21,6 @@ int prefer_symlink_refs;
 int is_bare_repository_cfg = -1; /* unspecified */
 int log_all_ref_updates = -1; /* unspecified */
 int warn_ambiguous_refs = 1;
-int unique_abbrev_extra_length;
 int repository_format_version;
 const char *git_commit_encoding;
 const char *git_log_output_encoding;