submodule: remove extra line feeds between callback struct and macro

submodule: remove extra line feeds between callback struct and macro

Many `submodule--helper` subcommands follow the convention that a struct
defines their callback data, and the declaration of that struct is
followed immediately by a macro to use in static initializers, without
any separating empty line.

Let's align the `init`, `status` and `sync` subcommands with that convention.

Mentored-by: Christian Couder <redacted>
Mentored-by: Kaartic Sivaraam <redacted>
Helped-by: Johannes Schindelin <redacted>
Helped-by: Philip Oakley <redacted>
Signed-off-by: Shourya Shukla <redacted>
Signed-off-by: Junio C Hamano <redacted>