Fix crash when using resetColorScheme()

Summary:
After the KNewStuff changes that were added to
EditProfileDialog::removeColorScheme(), now when
resetColorScheme() calls removeColorScheme() Konsole crashes.
Change resetColorScheme() to call ColorSchemeManager::deleteColorScheme()
directly; this is more logical since there's no need to delete the row
from the color schemes list as reseting the color scheme just deletes
the relevant color scheme from the user's home dir and "re-adds" the
original one from the system-wide location.

Reviewers: #konsole, hindenburg

Reviewed By: #konsole, hindenburg

Subscribers: #konsole

Tags: #konsole

Differential Revision: https://phabricator.kde.org/D11821

(cherry picked from commit 914d12a0)