MIPS: vmlinux.lds.S: remove duplicate _sdata symbol

_sdata is defined twice in vmlinux.lds.S. According to vmlinux.ld.h
in asm-generic, _sdata should be marked at the beginning RO_DATA_SECTION.

 *      _sdata = .;
 *      RO_DATA_SECTION(PAGE_SIZE)
 *      RW_DATA_SECTION(...)
 *      _edata = .;

Remove the one that is marked at RW_DATA_SECTION.

Signed-off-by: Tony Wu <tung7970@gmail.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/3215/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>