brontide: refer directly to the curve object in btcec's global namespace
brontide: refer directly to the curve object in btcec's global namespace

This commit modifies the `ecdh` function within the `brontide` package
to refer directly to the global curve params object in the `bcec`
package rather than reference it from the target public key. This
changes fixes a class of panics that have been uncovered recently but
*doesn’t* yet fix the root cause.

ddustin/lnd,aakselrod/lnd,ddustin/lnd,aakselrod/lnd,LightningNetwork/lnd,AndrewSamokhvalov/lnd,AndrewSamokhvalov/lnd,LightningNetwork/lnd