Remove runtime feature if no longer needed

[kulp]

Since #2166 was completed, there are no more instances of ::is_loaded() in the code for checking individual functions. At a minimum, I think this means we could stop using the clang-sys/runtime feature.

Does the lack of ::is_loaded() calls also mean that we could get rid of the rust-bindgen/runtime feature ?

Originally posted by @kulp in #2166 (comment)

[kulp]

This probably is not as worthwhile as I thought. Apart from cutting the load on CI in half, the amount of change to rust-bindgen source code seems pretty minimal, so it is not as if a lot of maintainability is to be gained.

I am closing this issue as not-very-important.