SplitView Sample -> `fix` the mismatch between rendered content and source code

[Zakariathr22]

Description

A simple update to resolve discrepancies between the rendered content and the displayed source code, the only change is:

• Remove the code that does not match from the displayed source code (SplitViewPage.xaml).

Motivation and Context

• The similarity between SplitView and NavigationView might be confusing for learners, so it would be best to remove it.
• fixes this issue: https://github.com/microsoft/WinUI-Gallery/issues/1587

How Has This Been Tested?

Manually Tested

Screenshots:

Types of changes

• [X] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)