Switch default implementation of QtcProcess from ProcessLauncher into QProcess

It looks like some changes into QProcess made it starting faster. These days, using Qt 6.4 branch, when starting a QtCreator, loading a session containing another QtCreator and switching to other session containing other QtCreator, the numbers are comparable. The fix was here: https://codereview.qt-project.org/c/qt/qtbase/+/417829.

The numbers are observed by running QtCreator with:
1. QTC_MEASURE_PROCESS= ./bin/qtcreator (for ProcessLauncher impl)
2. QTC_MEASURE_PROCESS= QTC_USE_QPROCESS= ./bin/qtcreator (for QProcess impl)

So, we should measure the numbers again and consider switching the default into using QProcess implementation.