Prevent trailing commas from breaking dak rm logic

This is all documented in #1114691 in the BTS

When we do a dak rm, dak will load the archive dependencies to detect breakage. It does this by SELECT-ing source_metadata (basically, the keys from a parsed deb822 dsc), source (the source version itself), and newest_src_association (newest source for a sutie).

This means that sources containing both Build-Depends and Build-Depends-Indep, and the first value to be returned contains a trailing comma, we wind up with two commas.

I'm sure everyone's guessed by now, but just to spell it out, While "foo," is accepted by the Python apt_pkg module, "foo,, bar" is not.

Since ",," is universally regarded as broken, we don't need to worry about multiple trailing commas, so a straight replace should solve this issue, even if it's a bit .... inelegant