qt5: revert QT_DIR_NAME name change

QT_DIR_NAME change was incorrectly included in another commit,
revert it.

Change-Id: I11b3453dac0fa4079cb7c1dafb0af0742e3a3404
Reviewed-by: Teemu Holappa <teemu.holappa@theqtcompany.com>

conf/distro/b2qt.conf

@@ -69,6 +69,4 @@ IMAGE_CLASSES += "image_types_sdcard deploy-conf"
 LICENSE_CREATE_PACKAGE = "1"
 COPY_LIC_MANIFEST = "1"
 
-QT_DIR_NAME = ""
-
 include conf/distro/include/${MACHINE}.conf