targets/rp2040: Remove extra .elf extension

Change-Id: I6591ea532d4ba88984428389105040daac20a6f3
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/110634
Pigweed-Auto-Submit: Armando Montanez <amontanez@google.com>
Reviewed-by: Anthony DiGirolamo <tonymd@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>

targets/rp2040/BUILD.gn

@@ -43,7 +43,6 @@ generate_toolchain("rp2040") {
   ]
   _toolchain_base = pw_toolchain_arm_gcc.cortex_m0plus_size_optimized
   forward_variables_from(_toolchain_base, "*", _excluded_members)
-  final_binary_extension = ".elf"
 
   # For now, no Pigweed configurations set up.
   defaults = {