cache.h: drop duplicate ensure_full_index() declaration

There are two identical declarations of `ensure_full_index()` in
cache.h.

Commit 3964fc2 ("sparse-index: add guard to ensure full index",
2021-03-30) provided an empty implementation of `ensure_full_index()`,
declaring it in a new file sparse-index.h. When commit 4300f84
("sparse-index: implement ensure_full_index()", 2021-03-30) fleshed out
the implementation, it added an identical declaration to cache.h.

Then 118a2e8 ("cache: move ensure_full_index() to cache.h",
2021-04-01) favored having the declaration in cache.h. Because of the
double declaration, at that point we could have just dropped the one in
sparse-index.h, but instead it got moved to cache.h.

As a result, cache.h contains the exact same function declaration twice.
Drop the one under "/* Name hashing */", in favor of the one under
"/* Initialize and use the cache information */".

Signed-off-by: Martin Ågren <[email protected]>
Acked-by: Victoria Dye <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: Martin Ågren

cache.h

@@ -350,8 +350,6 @@ void add_name_hash(struct index_state *istate, struct cache_entry *ce);
 void remove_name_hash(struct index_state *istate, struct cache_entry *ce);
 void free_name_hash(struct index_state *istate);
 
-void ensure_full_index(struct index_state *istate);
-
 /* Cache entry creation and cleanup */
 
 /*