Mk/bsd.port.mk: Remove confusing double "/" from _DISTDIR

_DISTDIR is a variable which stores the distfiles directory of the port
that is currently being worked on. In practice, it is usually just
the DISTDIR, although some ports also define a subdirectory. Currently,
a slash separating DISTDIR from DIST_SUBDIR is added to _DISTDIR
no matter if DIST_SUBDIR is set or not. This may cause confusion to
ports users by looking like a bug while it is not.

In particular, this patch casuses diagnostic logs of failed archive
extraction to look like this:

===>  Failed to extract "/portdistfiles/ttf-iosevka-ss05-4.1.1.zip".

Instead of this:

===>  Failed to extract "/portdistfiles//ttf-iosevka-ss05-4.1.1.zip".

PR: 282291
Reviewed by: pizzamig
Approved by: pizzamig (portmgr)
Event: Berlin Hackathon 202507
Differential Revision: https://reviews.freebsd.org/D27722