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

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