CLEANUP: peers: Remove unused static function `free_dcache_tx`

The function was added in commit 6c39198b575444466ddfc5368e481d2a7cf8111d,
but was also used within a single function `free_dcache` which was unused
itself.

see issue #301
see commit 10ce0c2f31ba06e52edfb52176dc47986314321b which removed
`free_dcache`
diff --git a/src/peers.c b/src/peers.c
index 52e2a8b..659992c 100644
--- a/src/peers.c
+++ b/src/peers.c
@@ -2851,13 +2851,6 @@
 	return NULL;
 }
 
-static void free_dcache_tx(struct dcache_tx *dc)
-{
-	free(dc->entries);
-	dc->entries = NULL;
-	free(dc);
-}
-
 /*
  * Allocate a cache of dictionary entries with <name> as name and <max_entries>
  * as maximum of entries.