remove block input support in merge (#7177)

# Description

Closes: #6937

# User-Facing Changes

N/A

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace --features=extra -- -D warnings -D
clippy::unwrap_used -A clippy::needless_collect` to check that you're
using the standard code style
- `cargo test --workspace --features=extra` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

crates/nu-command/tests/commands/merge.rs

@@ -30,7 +30,7 @@ fn row() {
             cwd: dirs.test(), pipeline(
             r#"
                 open caballeros.csv
-                | merge { open new_caballeros.csv }
+                | merge (open new_caballeros.csv)
                 | where country in ["Guayaquil Ecuador" "New Zealand"]
                 | get luck
                 | math sum