database: move striping of trailing '/' into helper function

Stripping trailing character is not that uncommon
operation. Particularly, the next patch has to perform it as
well. Lets move it to the separate function to avoid code duplication.

Also the new function has a little improvement: if the character to
strip is repeated several times in the end of a string, function
strips them all.

Signed-off-by: Yuri Volchkov <yuri.volchkov@gmail.com>