fix: unset imported config mappings before finding Python

[Illustar0]

I ran into a build failure when building plugin_python with RelWithDebInfo:

LINK : fatal error LNK1104: cannot open file 'Python::Module-NOTFOUND.obj'

CMake also reported CMP0111 warnings for Python::Module and Python::Python, indicating that their imported locations were not available for the RelWithDebInfo configuration.

The existing code temporarily clears CMAKE_MAP_IMPORTED_CONFIG_* before calling find_package(Python). This change uses unset() instead of assigning an empty string, so those variables are not defined while the Python imported targets are created.

With this change, the plugin_python build succeeds in my environment.

Tested with:

• CMake 4.4.3
• Python 3.13.15
• Visual Studio 2022 / v143
• RelWithDebInfo

I'm not sure whether this affects other CMake/Python combinations, but this seems to better match the intent of temporarily disabling the imported configuration mappings.