Remove thinlto servobuild.config option (#30830)

Rust uses a version of ThinLTO by default now [1]. This can be tweaked
by adjusting rust compiler flags, which is probably a better way of
controlling this than a custom servo configuration considering:

1. We want to remove the custom servo configuration eventually.
2. The -Z option that this configuration currently uses is unsupported
   by stable rust.

1. https://blog.rust-lang.org/inside-rust/2020/06/29/lto-improvements.html

python/servo/command_base.py

@@ -302,7 +302,6 @@ class CommandBase(object):
         self.config["build"].setdefault("ccache", "")
         self.config["build"].setdefault("rustflags", "")
         self.config["build"].setdefault("incremental", None)
-        self.config["build"].setdefault("thinlto", False)
         self.config["build"].setdefault("webgl-backtrace", False)
         self.config["build"].setdefault("dom-backtrace", False)
 
@@ -542,9 +541,6 @@ class CommandBase(object):
 
         env["CARGO_TARGET_DIR"] = servo.util.get_target_dir()
 
-        if self.config["build"]["thinlto"]:
-            env['RUSTFLAGS'] += " -Z thinlto"
-
         # Work around https://github.com/servo/servo/issues/24446
         # Argument-less str.split normalizes leading, trailing, and double spaces
         env['RUSTFLAGS'] = " ".join(env['RUSTFLAGS'].split())

servobuild.example

@@ -54,9 +54,6 @@ media-stack = "auto"
 #incremental = false
 #incremental = true
 
-# Whether to use ThinLTO or not
-#thinlto = false
-
 # Android information
 [android]
 # Defaults to the value of $ANDROID_SDK, $ANDROID_NDK, $ANDROID_TOOLCHAIN, $ANDROID_PLATFORM respectively