package/qt5/qt5base: do not change QMAKE_COMPILER in g++-base.conf

package/qt5/qt5base: do not change QMAKE_COMPILER in g++-base.conf

The variable QMAKE_COMPILER determines the compiler flavour (e.g.
gcc, clang, llvm, msvc etc.) and not a compiler path. Changing
it confuses the platform test of the qt5webengine package
(mkspecs/features/platform.prf):

  defineTest(isPlatformSupported) {
    QT_FOR_CONFIG += gui-private
    linux {
      if(!gcc:!clang)|intel_icc {
        skipBuild("Qt WebEngine on Linux requires clang or GCC.")
        return(false)
      }
      gcc:!clang:!isGCCVersionSupported(): return(false)
    }
  }

Revert the variable change introduced by commit 'package/qt5/qt5base: use
ccache for building host code' ([2]).

Fixes raspberrypi3_qt5we_defconfig/qt5webengine build ([1]):

  Qt WebEngine on Linux requires clang or GCC.
  QtWebEngine will not be built.

[1] https://gitlab.com/buildroot.org/buildroot/-/jobs/131217091
[2] https://git.buildroot.net/buildroot/commit/?id=c2ea056a1b34fd1354100d08e603b9374e225968

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>