Remove use of `std::numeric_limits<int>::max()` for beginRemoveRows in MergedProxyModel

[jonaski]

Remove use of std::numeric_limits<int>::max() in https://github.com/strawberrymusicplayer/strawberry/blob/c95886d8db863eb9a3a5617a79f6b6e02d25d4ec/src/core/mergedproxymodel.cpp#L127

Causing assert in debug:

22:57:39.523 ERROR unknown ASSERT: "last < rowCount(parent)" in file /home/jonas/Projects/qtbase/qtbase/src/corelib/itemmodels/qabstractitemmodel.cpp, line 2943