std, src, doc, test: remove unused variables

lib/std/unicode/throughput_test.zig

@@ -47,8 +47,6 @@ fn benchmarkCodepointCount(buf: []const u8) !ResultCount {
 pub fn main() !void {
     const stdout = std.io.getStdOut().writer();
 
-    const args = try std.process.argsAlloc(std.heap.page_allocator);
-
     try stdout.print("short ASCII strings\n", .{});
     {
         const result = try benchmarkCodepointCount("abc");