message: build: remove BIP70 configure option
  This was left in after so that anyone who had been compiling
  with (already disabled by default) BIP70 would realise that support
  had been completely removed in 0.20.0. However we should be able to
  remove it for 0.21.0.
diff: @@ -237,16 +237,6 @@ AC_ARG_ENABLE([zmq],
  [use_zmq=$enableval],
  [use_zmq=yes])
  -AC_ARG_ENABLE([bip70],
  - [AS_HELP_STRING([--enable-bip70],
  - [BIP70 (payment protocol) support in the GUI (no longer supported)])],
  - [enable_bip70=$enableval],
  - [enable_bip70=no])
  -
  -if test x$enable_bip70 != xno; then
  - AC_MSG_ERROR([BIP70 is no longer supported!])
  -fi
  -
  AC_ARG_WITH([libmultiprocess],
  [AS_HELP_STRING([--with-libmultiprocess=yes|no|auto],
  [Build with libmultiprocess library. (default: auto, i.e. detect with pkg-config)])],
  