fix(bifrost/zcash): remove unused isInRawMempool wrapper

!718 rerouted the mempool callers to isInRawMempoolWithErr, leaving the isInRawMempool bool wrapper (zcash client.go) with no callers. golangci-lint's unused analyzer flags it, which is currently failing the lint job on develop (e.g. job 15206567198) and consequently on every open MR/branch rebased onto develop (trunk --all reports it regardless of baseline).

This removes the dead wrapper; isInRawMempoolWithErr is the variant everything uses. No behavior change.

cc @zlyzol (from !718 (merged))