[Blackfin] arch: remove unused/incorrect definition

Signed-off-by: NMeihui Fan <mhfan@hhcn.com>
Signed-off-by: NMike Frysinger <vapier.adi@gmail.com>
Signed-off-by: NBryan Wu <cooloney@kernel.org>

include/asm-blackfin/bfin-global.h

@@ -125,7 +125,6 @@ extern unsigned long table_start, table_end;
 extern unsigned long bfin_sic_iwr[];
 extern u16 _bfin_swrst; /* shadow for Software Reset Register (SWRST) */
 extern struct file_operations dpmc_fops;
-extern char _start;
 extern unsigned long _ramstart, _ramend, _rambase;
 extern unsigned long memory_start, memory_end, physical_mem_end;
 extern char _stext_l1[], _etext_l1[], _sdata_l1[], _edata_l1[], _sbss_l1[],