microblaze: Remove duplicate declarations of _stext[] and _etext[]

They're already provided by <asm/sections.h>.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Michal Simek <monstr@monstr.eu>
Cc: microblaze-uclinux@itee.uq.edu.au
Signed-off-by: Michal Simek <michal.simek@xilinx.com>