rust: remove obselete opt-dist option

--llvm-shared is used only at one place: https://github.com/search?q=repo%3Arust-lang/rust%20shared_llvm&type=code. with --build-llvm=false it actually did nothing. no rebuild needed, just a cleanup

mingw-w64-rust/PKGBUILD

@@ -188,7 +188,6 @@ build() {
     local _pgo_opts=(--target-triple="$OSTYPE"
                      --checkout-dir="$PWD"
                      --llvm-dir="${MINGW_PREFIX}"
-                     --llvm-shared=false
                      --build-llvm=false
                      --python="${MINGW_PREFIX}/bin/python.exe"
                      --artifact-dir=opt-artifacts-${MSYSTEM}