don't check for doc/ dirs' existence

the doc/ dirs in the build dir won't be created until the docs have been
built, so of course checking whether they are there during the qmake
phase is counterproductive.

this also means that we'll get some complaints about non-existing
directories (for repos that don't create any docs). there is no
reasonable way to query qmake which repos are affected, and writing
shell-specific code to query it at make time seems a bit overengineered.

Task-number: QTBUG-38862
Change-Id: Ie0588e75bfc39718fffd46f0df6785428e396eb2
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Martin Smith <martin.smith@digia.com>

mkspecs/features/qt_docs.prf

@@ -25,8 +25,7 @@ QDOC += -outputdir $$QMAKE_DOCS_OUTPUTDIR
 DOC_INDEXES =
 !isEmpty(QTREPOS) {
     for(qrep, QTREPOS): \
-        exists($$qrep/doc): \
-            DOC_INDEXES += -indexdir $$qrep/doc
+        DOC_INDEXES += -indexdir $$qrep/doc
 } else {
     DOC_INDEXES += -indexdir $$[QT_INSTALL_DOCS/get]
 }