x86/tboot: remove dead declarations

These aren't needed anymore as of c9a4a1c419 ("x86/layout: Correct
Xen's idea of its own memory layout").

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/tboot.c
+++ b/xen/arch/x86/tboot.c
@@ -48,8 +48,6 @@ static uint64_t __initdata sinit_base, _
 #define TXTCR_HEAP_BASE             0x0300
 #define TXTCR_HEAP_SIZE             0x0308
 
-extern char __init_begin[], __bss_start[], __bss_end[];
-
 #define SHA1_SIZE      20
 typedef uint8_t   sha1_hash_t[SHA1_SIZE];