This commit removes the libuv and gyp submodules, as well as all build
infrastructure related to them.
For more context, see the [runtime removal
RFC](https://github.com/rust-lang/rfcs/pull/230)
[breaking-change]
Because the build system treats Makefile.in and the .mk files slightly
differently (.in is copied, .mk are included), this makes the system
more uniform. Fewer build system changes will require a complete
reconfigure.