generate: fix Loop proto compile error

[jamaljsr]

I noticed the daily CI deployment is failing due to a recent change in the loop repo. I was able to reproduce the issue locally by running ./generate.sh.

 Installing loop and loopd.
GO111MODULE=on go install -v -tags="" -ldflags "-X github.com/lightninglabs/loop.Commit=v0.28.9-beta-23-g2ec44f0c83a22756c7c20eeea94fe18e25386831" github.com/lightninglabs/loop/cmd/loop
GO111MODULE=on go install -v -tags="" -ldflags "-X github.com/lightninglabs/loop.Commit=v0.28.9-beta-23-g2ec44f0c83a22756c7c20eeea94fe18e25386831" github.com/lightninglabs/loop/cmd/loopd
~/dev/lightninglabs/lightning-api-ng
~/dev/lightninglabs/lightning-api-ng/build/protos/loop ~/dev/lightninglabs/lightning-api-ng
server.proto: File not found.
swapserverrpc/staticaddr.proto:2:1: Import "server.proto" was not found or had errors.
swapserverrpc/staticaddr.proto:72:14: "PrevoutInfo" is not defined.

This small change resolves the proto compilation error and will fix the CI deployment.

[djkazic]

AFAICT, this PR causes the looprpc docs to be dropped. I think we can rework it to bring it back in, experimenting locally.