Summary:
terminalpanel.cpp *should* compile on Windows, so there is no reason to
remove it from the build and use tons of #ifdefs in dolphinmainwindow.

We still keep the terminal panel disabled on Windows
(i.e. the two remaining #ifndef Q_OS_WIN in dolphinmainwindow),
because it is probably not functional.

But at least we won't break the Windows CI every time
someone touches the terminal panel code (see e.g. 2e942237).

Test Plan: Builds on Linux, someone on Windows should test this patch if possible.

Reviewers: #dolphin, #craft

Differential Revision: https://phabricator.kde.org/D10006