binaryplatforms.jl: Remove duplicated riscv64 entry (#56888)

Author: eschnett

base/binaryplatforms.jl

@@ -631,9 +631,6 @@ const arch_march_isa_mapping = let
             "a64fx" => get_set("aarch64", "a64fx"),
             "apple_m1" => get_set("aarch64", "apple_m1"),
         ],
-        "riscv64" => [
-            "riscv64" => get_set("riscv64", "riscv64")
-        ],
         "powerpc64le" => [
             "power8" => get_set("powerpc64le", "power8"),
         ],