Partially revert colors in tests

Author: hamishknight

test/lit.cfg

@@ -1955,10 +1955,6 @@ config.substitutions.append(('%cache-tool', config.swift_cache_tool))
 config.target_swift_frontend += " -typo-correction-limit 10 "
 subst_target_swift_frontend_mock_sdk += " -typo-correction-limit 10 "
 
-# Enable colors if we have them.
-if config.color_output:
-    config.target_swift_frontend += " -color-diagnostics "
-
 config.substitutions.append(('%module-target-triple',
                              target_specific_module_triple))
 config.substitutions.append(('%module-target-future', target_future))