Revert "Merge bitcoin#12870: make clean removes src/qt/moc_ files"

As noted by theuni and Sjors in bitcoin#12870, qt moc cleaning is handled
by CLEAN_QT via QT_MOC_CPP in Makefile.qt.include.

In my testing I configured, built and cleaned with qt4 and qt5 both,
absent the associated wildcard, and no MOC files were left after clean.
Propose we revert the change and reconsider if a specific file
is identified, then add that file to QT_MOC_CPP.

This reverts commit 1d54004, reversing
changes made to ad960f5.

Makefile.am

@@ -294,5 +294,6 @@ clean-docs:
 	rm -rf doc/doxygen
 
 clean-local: clean-docs
-	rm -rf coverage_percent.txt test_bitcoin.coverage/ total.coverage/ test/tmp/ cache/ $(OSX_APP) src/qt/moc_*
+	rm -rf coverage_percent.txt test_bitcoin.coverage/ total.coverage/ test/tmp/ cache/ $(OSX_APP)
 	rm -rf test/functional/__pycache__ test/functional/test_framework/__pycache__ test/cache
+