bts: Drop duplicated function to get trx by number

In the big mess of gsm_data we reached a point where we have multiple
functions doing the same thing, most probably because it's hard finding
stuff in there. Let's drop one of them (the one which less callers) and
move it to bts.*, where it belongs.

Change-Id: I9071a0ab250844619280fbe2be63ed99f2c87eb1
diff --git a/include/osmocom/bsc/bts.h b/include/osmocom/bsc/bts.h
index 07d2793..b38c11a 100644
--- a/include/osmocom/bsc/bts.h
+++ b/include/osmocom/bsc/bts.h
@@ -598,7 +598,7 @@
 
 int gsm_set_bts_type(struct gsm_bts *bts, enum gsm_bts_type type);
 
-struct gsm_bts_trx *gsm_bts_trx_by_nr(struct gsm_bts *bts, int nr);
+struct gsm_bts_trx *gsm_bts_trx_num(const struct gsm_bts *bts, int num);
 
 int bts_gprs_mode_is_compat(struct gsm_bts *bts, enum bts_gprs_mode mode);