Remove compiler-builtins-mangled-names

This config was added in 207de01 ("libary: Forward
compiler-builtins "asm"  and "mangled-names" feature") but it does not
appear this has ever been used. The PR adding it (RUST-78472) says that
this was exposed to help with configuration and points at the [Hermit
Cargo config], but as far as I can tell, this feature name has never
been mentioned in that repository's git history.

Thus, clean up a seemingly unneeded feature.

[Hermit Cargo config]: https://github.com/hermit-os/hermit-rs/blob/ab2b830930e6a9a98c8294997a8183feeabeda4a/.cargo/config

Author: tgross35

library/alloc/Cargo.toml

@@ -22,7 +22,6 @@ compiler_builtins = { path = "../compiler-builtins/compiler-builtins", features
 compiler-builtins-mem = ['compiler_builtins/mem']
 compiler-builtins-c = ["compiler_builtins/c"]
 compiler-builtins-no-f16-f128 = ["compiler_builtins/no-f16-f128"]
-compiler-builtins-mangled-names = ["compiler_builtins/mangled-names"]
 # Make panics and failed asserts immediately abort without formatting any message
 panic_immediate_abort = ["core/panic_immediate_abort"]
 # Choose algorithms that are optimized for binary size instead of runtime performance

library/std/Cargo.toml

@@ -98,7 +98,6 @@ panic-unwind = ["dep:panic_unwind"]
 compiler-builtins-c = ["alloc/compiler-builtins-c"]
 compiler-builtins-mem = ["alloc/compiler-builtins-mem"]
 compiler-builtins-no-f16-f128 = ["alloc/compiler-builtins-no-f16-f128"]
-compiler-builtins-mangled-names = ["alloc/compiler-builtins-mangled-names"]
 llvm-libunwind = ["unwind/llvm-libunwind"]
 system-llvm-libunwind = ["unwind/system-llvm-libunwind"]
 

library/sysroot/Cargo.toml

@@ -24,7 +24,6 @@ backtrace-trace-only = ["std/backtrace-trace-only"]
 compiler-builtins-c = ["std/compiler-builtins-c"]
 compiler-builtins-mem = ["std/compiler-builtins-mem"]
 compiler-builtins-no-f16-f128 = ["std/compiler-builtins-no-f16-f128"]
-compiler-builtins-mangled-names = ["std/compiler-builtins-mangled-names"]
 debug_refcell = ["std/debug_refcell"]
 llvm-libunwind = ["std/llvm-libunwind"]
 system-llvm-libunwind = ["std/system-llvm-libunwind"]