build: remove BIP70 configure option

This was left in after #17165, 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.

configure.ac

@@ -232,16 +232,6 @@ 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)])],