Revert "Use target-applies-to-host to avoid unnecessary cache invalid…

…ation"

This reverts commit 3d65c1e.
This was breaking our clippy lint configuration in .cargo/config.toml:
rust-lang/cargo#10744

.cargo/config.toml

@@ -1,13 +1,3 @@
-# Pass the rustflags specified to host dependencies (build scripts, proc-macros)
-# when a `--target` is passed to Cargo. Historically this was not the case, and
-# because of that, cross-compilation would not set the rustflags configured
-# below in `target.'cfg(all())'` for them, resulting in cache invalidation.
-#
-# Since this is an unstable feature (enabled at the bottom of the file), this
-# setting is unfortunately ignored on stable toolchains, but it's still better
-# to have it apply on nightly than using the old behavior for all toolchains.
-target-applies-to-host = false
-
 [alias]
 xtask = "run --package xtask --"
 
@@ -46,8 +36,5 @@ rustflags = [
     "-Wclippy::wildcard_imports",
 ]
 
-# activate the target-applies-to-host feature.
-# Required for `target-applies-to-host` at the top to take effect.
 [unstable]
 rustdoc-map = true
-target-applies-to-host = true