[libc++] NFC: Remove unused Lit features in the test suite

[libc++] NFC: Remove unused Lit features in the test suite

The libc++ test suite currently defines several features that are not
used anywhere in the tests, or that are redundant with other features.
For the purpose of simplifying config.py and to ease the bring up of a
new configuration, this commit removes some of these features:

- rename dylib-has-no-filesystem to c++filesystem-disabled, which exists
- rename apple-darwin to just darwin, which is already set
- remove useless setting of libstdc++, which is already set correctly
- remove libcpp-abi-unstable, which is not used anywhere
- remove the glibc-XXX features, which are not used anywhere