nios2: Remove binutils support for Nios II target.

nios2: Remove binutils support for Nios II target.

The Nios II architecture has been EOL'ed by the vendor.  This patch
removes all binutils, bfd, gas, binutils, and opcodes support for this
target with the exception of the readelf utility.  (The ELF EM_*
number remains valid and the relocation definitions from the Nios II
ABI will never change in future, so retaining the readelf support
seems consistent with its purpose as a utility that tries to parse the
headers in any ELF file provided as an argument regardless of target.)