compiler_builtins rustc-dep-of-std/core removal

currently, we have a few crates packaged that are not actually for consumption by regular crates, but just for building libstd/libcore as part of the toolchain build. since the toolchain build uses vendored crates and not the ones from debcargo-conf, we can and should remove these crates:

• compiler_builtins
• rustc-std-workspace-alloc
• rustc-std-workspace-core
• rustc-std-workspace-std

to achieve this, we also need to remove the corresponding features which only exist to support this usecase: rustc-dep-of-alloc / rustc-dep-of/core / rustc-dep-of-std

these (additional) crates currently include one of those features or pull in one of the crates listed above:

• librust-adler+rustc-dep-of-std-dev
• librust-adler2-dev
• librust-backtrace-sys+rustc-dep-of-std-dev
• librust-rustix-dev
• librust-bitflags-dev (needs to wait for rustix)
• librust-linux-raw-sys-dev (needs to wait for rustix)
• librust-bitflags-1-dev
• librust-cfg-if-dev
• librust-cfg-if-0.1-dev (slated for RM already)
• librust-getopts-dev
• librust-getrandom-dev
• librust-libc-dev (needs to wait for getrandom)
• librust-memchr-dev
• librust-rustc-demangle+rustc-dep-of-std-dev
• librust-hashbrown-0.14-dev (missing changelog commit in git?)
• librust-hashbrown-dev