The Boston-based data backup company Carbonite became one of a growing number of companies that have pulled advertising from Rush Limbaugh’s talk show after the conservative host called a Georgetown University Law student a “slut’’ and a prostitute last week for supporting expanded access to birth control. Carbonite chief executive David Friend said on the company website that Limbaugh’s comments overstepped any reasonable bounds of decency. The Associated Press reported Sunday that a total of seven advertisers have withdrawn their spots from the radio program. Limbaugh apologized on Saturday for his remarks, which drew the ire of Democrats and Republicans alike. The apology was not enough for Friend, who said he hopes to contribute to more civilized public discourse.