Removed dual calenders in build_classes screen

[jaivsh]

Description

I've removed the dual calenders issue in the build_classes.dart screen. Also, I adjusted the UI opf the screen for single calender display.

Fixes #419

Have you read the Contributing Guidelines on Pull Requests?

• Yes
• No

Type of change

• 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 not work as expected)

Checklist:

• My code follows the style guidelines(Clean Code) of this project
• I have performed a self-review of my own code
• My changes generate no new warnings
• I have added tests/screenshots(if any) that prove my fix is effective or that my feature works.

[jaivsh]

@avinashkranjan please review this PR!