don't log method names to stderr unless --grpc-logging-insecure

[LarryRuane]

Previously, each method calls causes the following line to be logged to /dev/stderr (note, this is different from the other logging).

INFO[0007] method called    duration=5.026098ms error="<nil>" method=/cash.z.wallet.sdk.rpc.CompactTxStreamer/GetLightdInfo peer_addr="127.0.0.1:48274"

This PR removes this logging unless the --grpc-logging-insecure command-line argument is given to lightwalletd. This will prevent log files from overflowing in the default situation (must be explicitly enabled).