Clean up sqlite3 further in CMake

[ErixenCruz]

#3987 changed the CMake to link to the explicit target SQLite::SQLite3 and find SQLite3, which makes use of CMake's built-in FindSQLite3 script instead of PROJ's.

@mwtoews mentioned in the CMake 3.16 upgrade PR that there are some Sqlite3 configuration variables that should probably be adjusted or deprecated. #3997 (comment). Reading about find_path, which is called in the built-in FindSQLite3 script, the configuration variables might just have to be mapped to SQLite3_ROOT. A CMake expert may chime in here.

The other thing is that we can probably remove PROJ's FindSqlite3 and rely entirely on built-in CMake.

[mwtoews]

Thanks @ErixenCruz , that's correct, I'm planning to replace cmake/FindSqlite3.cmake with the FindSQLite3 built-in. Everybody [usually] wins when bespoke/custom modules are replaced with better-known built-in modules.