How to remove single quotes from a string using sed

If I need to remove the following line from certain files
ob_start('ffggg_ggg');

I have tried
grep -rl "ob_start('ffggg_ggg');" /pathtosearch | xargs sed -i 's/[ob_start('ffggg_ggg');]//g'

but the rest of the characters have been removed except for single quotes.

How can I remove single quotes from a string using sed command?