chore: remove redundant pageobject feature

[laolubenson]

Change list

Please provide briefly described change list which are you going to propose.

Types of changes

What types of changes are you proposing/introducing to .NET client?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• New feature (non-breaking change which adds value to the project)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Documentation

• Have you proposed a file change/ PR with appium to update documentation?

This can be done by navigating to the documentation section on http://appium.io selecting the appropriate command/endpoint and clicking the 'Edit this doc' link to update the C# example

Integration tests

• Have you provided integration tests to pass against the beta version of appium? (for Bugfix or New feature)

Details

Removes support for redundant SeleniumExtras code (containing the necessary Page object feature components). Removes the dependency to an unmaintained repository, unlinks this repository to selenium v3, and enables full .netStandard/core support.

The pageobject feature may return with .netStandard support in the future but will have to be a separate repo. Removing this code will prevent users from raising issues which lie in code that doesn't belong to us thus closes #444 #436 #385 #205 #179 #163 #121

[mykola-mokhnach]

Could you please provide a proper issue description? It is not very clear to me why this change is needed

[laolubenson]

@mykola-mokhnach done

[laolubenson]

A force push merged this in. Build succeeded