Remove hard dependency on libjpeg on Linux.

It made the build fail on systems that use libjpeg-turbo.

Task-number: QTBUG-48606
Change-Id: I1aa6acab1627d3f4613b4ca39837b6deb25d3cc0
Reviewed-by: Milian Wolff <milian.wolff@kdab.com>

tools/qmake/mkspecs/features/configure.prf

@@ -29,7 +29,6 @@ defineTest(runConfigure) {
         contains(QT_CONFIG, xcb): REQUIRED_PACKAGES += libdrm xcomposite xcursor xi xrandr xscrnsaver xtst
         contains(QT_CONFIG, pulseaudio): REQUIRED_PACKAGES += libpulse
         contains(QT_CONFIG, system-png): REQUIRED_PACKAGES += libpng
-        contains(QT_CONFIG, system-jpeg): REQUIRED_PACKAGES += libjpeg
         contains(QT_CONFIG, system-harfbuzz): REQUIRED_PACKAGES += harfbuzz
         !cross_compile: REQUIRED_PACKAGES += libpci nss