cmake: Remove asset hashing, CMAKE_ASSETS_DIR

Simplifies build setup for cmake.

• By default, SRB2_CONFIG_ASSET_DIRECTORY is set to an empty string. It will be ignored, and assets will not be added to the install target.
• When set, SRB2_CONFIG_ASSET_DIRECTORY will be resolved to an absolute path, and a predetermined list of files, and the models subdirectory in it, will be added to the install target only for macOS and Windows.
• The code to use the old SRB2_ASSET_DIRECTORY variable in WAD loading is removed so the path never shows up in built executables. Instead, set the SRB2WADDIR env var when running the game, or copy the build output to your game directory on Windows.