Remove unused execution_rpc CLI parameter from opstack server

[sashaodessa]

PR Description

Removes unused execution_rpc CLI parameter from the opstack server binary that was declared but never used.

Problem

The execution_rpc field in the Cli struct was defined as a required CLI argument but was never referenced in the code. This created a misleading interface where users could pass a flag that would be silently ignored.

Solution

• Removed the unused execution_rpc: Url field from the Cli struct
• CLI interface now accurately reflects the actual functionality

Impact

• Improves CLI usability by removing confusing unused parameters
• Reduces technical debt
• No breaking changes (parameter was unused)