Removing a platform is cleaned up in dist but not rustup

[mstine]

Steps to reproduce:

1. Configure mirror.toml:

platforms_windows = [
     "x86_64-pc-windows-msvc",
     "x86_64-pc-windows-gnu",
]

2. Sync
3. Remove a platform:

platforms_windows = [
     "x86_64-pc-windows-msvc",
]

4. Sync

Result: *-gnu files are removed from my-mirror/dist but not from my-mirror/rustup.