disas/nios2: Simplify endianess conversion

Since commit 12b6e9b2 ("disas: Clean up CPUDebug initialization")
the disassemble_info->bfd_endian enum is set for all targets in
target_disas(). We can directly call print_insn_nios2() and simplify.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20210807110939.95853-3-f4bug@amsat.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>