object-file: drop check for inflight transactions

object-file: drop check for inflight transactions

ODB transactions are started via `odb_transaction_begin()` and contain
validation to avoid starting multiple transactions at the same time. The
"files" backend also has the same logic, but is redundant due to the
generic layer already handling it. Drop this validation from the "files"
backend accordingly.

Signed-off-by: Justin Tobler <redacted>
Signed-off-by: Junio C Hamano <redacted>