Remove/Update newtonsoft unity DLLs

[sacOO7]

• As per https://github.com/jilleJr/Newtonsoft.Json-for-Unity#-deprecation-warning, we don't need to include Newtonsoft DLLs anymore in the unity package.
• This package is already being maintained and published by Unity.
• Relevant bug fixes are being offered by Unity for the same.
• Need to check if the removal of DLLs can cause incompatibility issues.
• Not a high priority issue, since workaround for dll conflicts is well documented in the unity sdk doc

┆Issue is synchronized with this Jira Task by Unito

[sync-by-unito[bot]]

➤ Automation for Jira commented:

The link to the corresponding Jira issue is https://ably.atlassian.net/browse/SDK-3639