Improve code quality for SkyWallet contrib plugin for v1

[Olemis Lang]

Is your feature request related to a problem? Please describe.
SkyWallet signer PR #253 for #139 was rushed to meet v0.27.0 deadline. Some code quality items were not enforced.

Describe the solution you'd like

• Rename Remove PersistibleSet
• Wrong condition check
• FIXME comment in walleModel.data
• Revert type of WalletDirectory.wltService , core.PersistibleSet=> SkycoinLocalWallet
• Remove WalletDirectory.SetWltService

Additional context
Original work developed in PR #253 for #139 . More details in open review