Extra column in csv export when sponsorship_total >= 1000

Bug introduced by commit a786f9badbccd329e5acec46d2f715219b9e99fc.

Due to the use of number_format() function with default parameters, PHP
outputs the sponsorship amount formatted with ',' as a thousands
separator, which also happens to be MantisBT's default csv separator.
Since the fields are not escaped, this causes an extra column to be
generated.

Call to number_format() was removed, as it makes more sense to output
raw data in csv exports.

Fixes 0013958