version: stop using the_repository

Actually it has never been used in version.c since cf7ee48190 (agent:
advertise OS name via agent capability, 2025-02-15) added the dependency
macro.  Remove it, along with the also unused struct declaration.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

version.c

@@ -1,5 +1,3 @@
-#define USE_THE_REPOSITORY_VARIABLE
-
 #include "git-compat-util.h"
 #include "version.h"
 #include "strbuf.h"

version.h

@@ -1,8 +1,6 @@
 #ifndef VERSION_H
 #define VERSION_H
 
-struct repository;
-
 extern const char git_version_string[];
 extern const char git_built_from_commit_string[];