Don't double format spreadsheet currency values with custom format strings.

Summary: If there is a format string for a currency value only format the number and let the format string append or prepend the appropriate currency symbol.

Reviewers: pvuorela

Reviewed By: pvuorela

Subscribers: Calligra-Devel-list

Tags: #calligra:_3.0

Differential Revision: https://phabricator.kde.org/D25036