Should I remove a 7 CM asymptomatic subserosal fibroid before trying to get conceive? I have one ovary due to a borderline cystic tumor in the past.

I don't think so. The subserosal "fibroids" aren't usually the ones that interfere with conception.
More information. Your oby/ gyne will be better suited to advice. Fibroids do contribute to fetal loss and may interfere with implantation. In some cases it is prudent to remove especially if symptomatic or contributing to fertility issues.