Default compiler bootstrap config causes a warning to be shown

[yotamofek]

The enable-warnings = true line in src/bootstrap/defaults/bootstrap.compiler.toml causes the following warning to be shown every time x.py is invoked:

WARNING: `llvm.enable-warnings` has no effect with `llvm.download-ci-llvm`. Current value: Some(true), Expected value(s): None

Not sure what the correct fix here is, so opening up this issue instead of filing a PR to fix it :)

[onur-ozkan]

The enable-warnings = true line should be removed.