build: remove unnecessary qt xcb patching
build: remove unnecessary qt xcb patching

The lines that this sed command was modifying were removed some time
ago. Relevant upstream change:

https://code.qt.io/cgit/qt/qtbase.git/commit/src/plugins/platforms/xcb/qxcbxsettings.cpp?id=538b9f504c0de11c473a40aed66df9900ac1c6c4

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