examples: rp-pico: Remove -Cinline-threshold

This flag was deprecated and no longer has any effect.  The example
works fine without perfectly tuned inlining, so let's drop it without
replacement.

Link: rust-lang/rust#124712

examples/rp-pico/.cargo/config.toml

@@ -16,7 +16,6 @@ rustflags = [
   #   trap unreachable can save a lot of space, but requires nightly compiler.
   #   uncomment the next line if you wish to enable it
   # "-Z", "trap-unreachable=no",
-  "-C", "inline-threshold=5",
   "-C", "no-vectorize-loops",
 ]