KCharSelect: remove unnecessary call

Summary:
d->_k_sectionSelected() refills the block combo which automatically
sets the current index to 0 (from -1 set by clear) and calls
_k_blockSelected.

Doing it twice is actually expensive since an item model is deleted
and recreated in that method.

Test Plan:
ctest (the unittest in the previous patch actually requires
this fix, due to the QHeaderView asserts)

Reviewers: cfeck

Reviewed By: cfeck

Differential Revision: https://phabricator.kde.org/D20516