NFC: Remove swift_indent_main.cpp

`swift-indent` has already been removed, this code
is unused.

Author: hamishknight

lib/DriverTool/CMakeLists.txt

@@ -11,7 +11,6 @@ set(driver_sources_and_options
                 modulewrap_main.cpp
                 swift_api_digester_main.cpp
                 swift_cache_tool_main.cpp
-                swift_indent_main.cpp
                 swift_symbolgraph_extract_main.cpp
                 swift_parse_test_main.cpp)