config: remove unused Rust 1.71 build environment

All Rust branches have been moved to Rust 1.72.0, thus the build
environment for the previous version is no longer needed.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

config/core/build-configs.yaml

@@ -712,12 +712,6 @@ build_environments:
     cc_version: 17
     arch_params: *clang_15_arch_params
 
-  rustc-1.71:
-    cc: clang
-    cc_version: 16
-    arch_params:
-      x86_64: *x86_64_params
-
   rustc-1.72:
     cc: clang
     cc_version: 16