build: remove `FindSQLite3` cmake module (NFC)
build: remove `FindSQLite3` cmake module (NFC)

This was introduced in CMake 3.13, and we require CMake 3.15.  Remove
the local copy of the module.

apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild