NIOS2: setup.c: drop unused variable 'dram_start'

This is a nuisance when CONFIG_WERROR is set, so drop the variable
declaration since the code that used it was removed.

../arch/nios2/kernel/setup.c: In function 'setup_arch':
../arch/nios2/kernel/setup.c:152:13: warning: unused variable 'dram_start' [-Wunused-variable]
  152 |         int dram_start;

Fixes: 7f7bc20b ("nios2: Don't use _end for calculating min_low_pfn")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>
Cc: Andreas Oetken <andreas.oetken@siemens.com>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>