Modify existing services to leverage locale handling

Expected behavior

The changes in #78 support more consistent locale handling within WordPressKit, but a few services still explicitly set the locale parameter.

Actual behavior

At the time of this writing, the following services set locale explicitly:

• AccountServiceRemoteREST
• ActivityServiceRemote - two methods
• PlanServiceRemote
• PlanServiceRemote_ApiVersion1_3
• ReaderSiteSearchServiceRemote
• TimeZoneServiceRemote
• WordPressComServiceRemote

We should be able to remove this and rely on the implicit framework behavior.

Steps to reproduce the behavior

N/A

WordPressKit Environment

All devices & versions of WordPressKit, Xcode & iOS.

• WordPressKit Version [e.g. 22] : all versions
• OS: [e.g. iOS 12.1.3 (16D40)] : all versions
• Device: [e.g. iPhone XS] : all devices
• Xcode Version: [e.g. Xcode 10.1 (10B61)] : all versions

Additional Context

N/A