Just avoiding two compiler warnings

[remkos]

Description of proposed changes

Changed two source files to avoid compiler warnings.

• gmt_remote.c: removed unused variable.
• grdview.c: cast input argument to curl_easy_setopt as long, as is required.

No bug fixes, no new functionalities.

Reminders

• Make sure that your code follows our style. Use the other functions/files as a basis.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Describe changes to function behavior and arguments in a comment below the function declaration.
• If adding new functionality, add a detailed description to the documentation and/or an example.