Remove redundant error handling from asURLEncodedData

See discussion at
https://github.com/wordpress-mobile/WordPressAuthenticator-iOS/pull/717/files#r1045479156

Author: mokagio

WordPressAuthenticator/OAuth/OAuthError.swift

@@ -3,18 +3,10 @@ enum OAuthError: LocalizedError {
     // ASWebAuthenticationSession
     case inconsistentWebAuthenticationSessionCompletion
 
-    // OAuth token request
-    case failedToBuildURLQuery
-    case failedToEncodeURLQuery(query: String)
-
     var errorDescription: String {
         switch self {
         case .inconsistentWebAuthenticationSessionCompletion:
             return "ASWebAuthenticationSession authentication finished with neither a callback URL nor error"
-        case .failedToBuildURLQuery:
-            return "Failed to build URL query string"
-        case .failedToEncodeURLQuery(let query):
-            return "Failed to encode URL query string '\(query)'"
         }
     }
 }

WordPressAuthenticator/OAuth/OAuthTokenRequestBody.swift

@@ -33,14 +33,7 @@ struct OAuthTokenRequestBody: Encodable {
         var components = URLComponents()
         components.queryItems = items
 
-        guard let query = components.query else {
-            throw OAuthError.failedToBuildURLQuery
-        }
-
-        guard let data = query.data(using: .utf8) else {
-            throw OAuthError.failedToEncodeURLQuery(query: query)
-        }
-
-        return data
+        // We can assume `query` to never be nil because we set `queryItems` in the line above.
+        return Data(components.query!.utf8)
     }
 }