Toolchain: Remove dependency on gnu-sed for Clang on Darwin

Author: https://github.com/EWouters
Commit: 05d8e2f6f8
Pull-request: https://github.com/SerenityOS/serenity/pull/21031
Reviewed-by: https://github.com/ADKaster
Reviewed-by: https://github.com/juniorrantila

Toolchain/BuildClang.sh

@@ -36,7 +36,6 @@ elif [ "$SYSTEM_NAME" = "Darwin" ]; then
     MD5SUM="md5 -q"
     REALPATH="grealpath"  # GNU coreutils
     INSTALL="ginstall"    # GNU coreutils
-    SED="gsed"            # GNU sed
 fi
 
 NPROC=$(get_number_of_processing_units)