build, qt: No longer need to set QT_RCC_TEST=1 for determinism

Since Qt 5.3.1 hash seeding is disabled for rcc.
See commit 5283a6c87beac5a43f612786fefd6e43f2c70bf6.

contrib/guix/libexec/build.sh

@@ -175,7 +175,6 @@ case "$HOST" in
 esac
 
 # Environment variables for determinism
-export QT_RCC_TEST=1
 export QT_RCC_SOURCE_DATE_OVERRIDE=1
 export TAR_OPTIONS="--owner=0 --group=0 --numeric-owner --mtime='@${SOURCE_DATE_EPOCH}' --sort=name"
 export TZ="UTC"