By JACOB ADELMAN Home sales in the San Francisco Bay Area plummeted 22.8 percent last month from the previous year to reach their lowest level in 15 years, a tracking firm reported Thursday.San Diego-based MDA DataQuick said the drop from 8,771 homes in July 2009 to 6,773 homes last month came as the market adjusted to the end of federal tax credits for first-time buyers.Last month was the slowest July since 1995, when just under 6,666 homes were sold in the nine-county region, the firm said.