message: doc: Drop outdated note
  It is outdated since bitcoin/bitcoin#23060.
diff: @@ -79,9 +79,6 @@ For linux RISC-V 64-bit cross compilation (there are no packages for 32-bit):
  sudo apt-get install g++-riscv64-linux-gnu binutils-riscv64-linux-gnu
  -RISC-V known issue: gcc-7.3.0 and gcc-7.3.1 result in a broken `test_bitcoin` executable (see https://github.com/bitcoin/bitcoin/pull/13543),
  -this is apparently fixed in gcc-8.1.0.
  -
  For linux S390X cross compilation:
  sudo apt-get install g++-s390x-linux-gnu binutils-s390x-linux-gnu
  