remove deprecated rustfmt config: merge_imports

Summary:
`merge_imports = true` is deprecated and has already been replaced by `imports_granularity = "Item"`

This is just a clean up diff so that `cargo fmt` does not give warnings about deprecated field usage.

Reviewed By: ndmitchell

Differential Revision: D37326838

fbshipit-source-id: 2e8ace2f7d4040308e61b30bf2f8db57fd7a612b

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"