update defaultNetwork to mainnet

[dstadulis]

Background

The demon use the testnet network by default

taproot-assets/tapcfg/config.go

Line 51 in 34f3283

defaultNetwork = "testnet"

Due to
d78e93b
in #567

We can either change the default to mainnet now, to reflect that the alpha is compatible with mainnet, or wait until we update tag from alpha to beta.

Users can be confused about having to set the --network=mainnet CLI flag

Issue will be satisfied by:

• Revert d78e93b