Remove the assertion so that an error is returned for invalid bech32.
An error is preferable to crashing the entire node if there's an extra
"lightning:" prefix:
$ lightning-cli pay "lightning:lightning:"
Node log:
pay: common/bolt11.c:718: bolt11_decode_nosig: Assertion `!has_lightning_prefix(str)' failed.
pay: FATAL SIGNAL 6
...
INFO plugin-pay: Killing plugin: exited during normal operation
**BROKEN** plugin-pay: Plugin marked as important, shutting down lightningd