spec-update: get rid of max-witness-len

We can use a fixed value and close the channel if they don't cover their
amount; this wasn't really helping with anything other than setting a
floor for an expected feerate

wire/extracted_peer_experimental_dual_fund

@@ -1,6 +1,6 @@
 --- wire/extracted_peer_wire_csv	2020-07-28 12:36:12.063168014 -0500
-+++ -	2020-08-24 19:16:50.026185750 -0500
-@@ -31,6 +31,47 @@
++++ -	2020-08-31 21:00:40.856646471 -0500
+@@ -31,6 +31,46 @@
  tlvdata,n2,tlv1,amount_msat,tu64,
  tlvtype,n2,tlv2,11
  tlvdata,n2,tlv2,cltv_expiry,tu32,
@@ -11,7 +11,6 @@
 +msgdata,tx_add_input,prevtx,byte,prevtx_len
 +msgdata,tx_add_input,prevtx_vout,u32,
 +msgdata,tx_add_input,sequence,u32,
-+msgdata,tx_add_input,max_witness_len,u16,
 +msgdata,tx_add_input,redeemscript_len,u16,
 +msgdata,tx_add_input,script,byte,redeemscript_len
 +msgdata,tx_add_input,tlvs,tx_add_input_tlvs,
@@ -48,7 +47,7 @@
  msgtype,open_channel,32
  msgdata,open_channel,chain_hash,chain_hash,
  msgdata,open_channel,temporary_channel_id,byte,32
-@@ -82,6 +123,54 @@
+@@ -82,6 +122,54 @@
  msgtype,funding_locked,36
  msgdata,funding_locked,channel_id,channel_id,
  msgdata,funding_locked,next_per_commitment_point,point,
@@ -103,7 +102,7 @@
  msgtype,shutdown,38
  msgdata,shutdown,channel_id,channel_id,
  msgdata,shutdown,len,u16,
-@@ -169,6 +258,11 @@
+@@ -169,6 +257,11 @@
  msgdata,channel_update,fee_base_msat,u32,
  msgdata,channel_update,fee_proportional_millionths,u32,
  msgdata,channel_update,htlc_maximum_msat,u64,,option_channel_htlc_max