Implemented File Removal

Implemented file removal in ospfs_unlink().
File Operations Fix

Fixed file operations:
- Added indirect range case in direct_index().
- Added proper reset during allocation failure in add_block().
- Added under and overflow cases in add_block().
- Used n-1 index references in remove_block().
- Added under and overflow cases in remove_block().

All 17 test cases now pass successfully.