buck2: remove merge_imports from rustfmt config as it is deprecated (#1)

Summary:
Pull Request resolved: #1

Seeing a lot of warnings when running `cargo fmt`,
```
Warning: the `merge_imports` option is deprecated. Use `imports_granularity="Crate"` instead
```

[GitHub issue on the deprecation](rust-lang/rustfmt#3362 (comment))

As we already have the imports_granularity="Item", it seems we can get rid of the deprecated option?

Reviewed By: ndmitchell

Differential Revision: D37313506

fbshipit-source-id: ce2da68de2dac15df5df3e6803be67baaa42cb15

Author: themarwhal

rustfmt.toml

@@ -1,7 +1,6 @@
 edition = "2021"
 version = "Two"
 imports_granularity = "Item"
-merge_imports = true
 use_field_init_shorthand = true
 use_try_shorthand = true
 group_imports = "StdExternalCrate"