build: remove duplicate data directory installation in cmake

[scarf005]

Purpose

CMake was installing data/ twice:

1. Main CMakeLists.txt: install(DIRECTORY data ...)
2. data/CMakeLists.txt: explicit subdirectory list

Prerequisite for Flathub CMake migration.

Solution

1. Removed data/ install from main CMakeLists.txt
2. Simplified data/CMakeLists.txt to copy entire directory with exclusions

Verification

# System install (USE_PREFIX_DATA_DIR=ON)
ls /tmp/cbn-test-system/share/cataclysm-bn/
# → font/ json/ lua/ mods/ etc. (no XDG/, no CMakeLists.txt)

# Portable install (USE_PREFIX_DATA_DIR=OFF)  
ls /tmp/cbn-test-portable/data/
# → font/ json/ lua/ mods/ etc. (no XDG/, no CMakeLists.txt)

Both modes work correctly.

[RobbieNeko]

Happy to see that this should let us drop the bandaid fixes in the future, in particular for whenever next stable is made. For now, the CMake migration in the flatpak can (likely) go on thanks to bandaid fixes.