firmware: fix compilation with arm-none-eabi-gcc 10.1.0

These symbols are defined, but never used:

  - struct last_rach - seems to be copy-pasted from prim_rach.c,
  - tall_msgb_ctx - already defined in libosmocore.

Change-Id: I6077c8e9b441f7848d1a4c25a8b5e1aed82f4b7d

src/target/firmware/comm/msgb.c

@@ -32,8 +32,6 @@
 
 #define NO_TALLOC
 
-void *tall_msgb_ctx;
-
 #ifdef NO_TALLOC
 /* This is a poor mans static allocator for msgb objects */
 #define MSGB_DATA_SIZE	256+4

src/target/firmware/layer1/prim_freq.c

@@ -49,11 +49,6 @@
 
 #include <l1ctl_proto.h>
 
-struct {
-	uint32_t fn;
-	uint16_t band_arfcn;
-} last_rach;
-
 /* if the "starting time" is reached, use frequencies "after time" */
 static int l1s_freq_cmd(__unused uint8_t p1, __unused uint8_t p2, __unused uint16_t p3)
 {