Get rid of bts singletons

There's no BTS single global object anymore, get rid of those APIs. Move
users to use "pcu->bts", which will evolve to a linked list in the
future.

Change-Id: I9cf762b0d3cb9e2cc3582727e07fa82c8e183ec5
diff --git a/src/osmobts_sock.cpp b/src/osmobts_sock.cpp
index 6addda6..d94c7e7 100644
--- a/src/osmobts_sock.cpp
+++ b/src/osmobts_sock.cpp
@@ -62,7 +62,7 @@
 
 static void pcu_tx_txt_retry(void *_priv)
 {
-	struct gprs_rlcmac_bts *bts = bts_main_data();
+	struct gprs_rlcmac_bts *bts = the_pcu->bts;
 
 	if (bts->active)
 		return;
@@ -90,7 +90,7 @@
 static void pcu_sock_close(int lost)
 {
 	struct osmo_fd *bfd = &pcu_sock_state.conn_bfd;
-	struct gprs_rlcmac_bts *bts = bts_main_data();
+	struct gprs_rlcmac_bts *bts = the_pcu->bts;
 	uint8_t trx, ts;
 
 	LOGP(DL1IF, LOGL_NOTICE, "PCU socket has %s connection\n",