Makefile: drop -j option from dpkg-buildpackage

Makefile: drop -j option from dpkg-buildpackage

From man dpkg-buildpackage:

> -j, --jobs[=jobs|auto]
> Specifies the number of jobs allowed to be run simultaneously (since
> dpkg 1.14.7, long option since dpkg 1.18.8). The number of jobs
> matching the number of online processors if auto is specified (since
> dpkg 1.17.10), or unlimited number if jobs is not specified. The
> default behavior is auto (since dpkg 1.18.11) in non-forced mode
> (since dpkg 1.21.10), and as such it is always safer to use with any
> package including those that are not parallel-build safe.

The option was added in the Makefile by commit 4ba321f ("build qemu
multithreaded") which states:

> same as in pve-kernel where we have --jobs=auto

But according to the man page, -j without an argument is not the same
and means unlimited. Using the number of online cores seems more
sensible and was the original intention. Again, according to the man
page, the default is auto since dpkg 1.18.11 (or Debian Stretch), so
just drop the option.

The motivation to look into this was that after the recent upstream
commit d1ce2cc95b ("Makefile: preserve --jobserver-auth argument when
calling ninja") having -j as the make flag would be broken as it was
mistakenly passed to ninja (for which the argument for -j is not
optional). Should get fixed soon [0].

[0]: https://lore.kernel.org/qemu-devel/20240412100401.20047-2-pbonzini@redhat.com/T/#u

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>